Traditional Korean medicinal system means the Constitution Medicine from Korea, of which another expression is Sasang constitutional Typology.

 Background on Constitution Medicine
A brand new science was launched by Lee Jema (1836-1900) a century ago in the latter part of the Korean Chosun Dynasty.
In 1894 Lee Jema asserted in his book Dongyi Soose Bowon (Longevity and Life Preservation in Oriental Medicine), that each person is born with a different organ structure and therefore have different characters, temperaments, and physiological and pathological phenomena. He asserted that even though people have the same illness, they need different medicines depending on their respective natural organ structures.

This assertion of his was based on his own experience.  Even though he was a medical doctor, he suffered from a chronic illness.  He tried everything to cure himself, but was unable to heal himself.  He cured other people with similar symptoms, but the same medicine was ineffective on himself.  This lead him to repeated research into the reason, and he came to realize that, in contrast to the others, he had a rare constitution, taeyangin, so the cures for others couldn't work on him.  Lee Jema finally cured himself, creating his own recipe appropriate to his constitution, and with that personal experience he was able to work out the theory of constitution medicine.

His book Dongyi Soose Bowon has the reasons for the deficiency and excess syndromes of the organs, which vary with the individual; the way to distinguish the different constitutions by physiological and pathological characteristics and figure; the list of medicines and prescriptions classified by him according to the constitutions; and his experiences in practical healing.

In its time, that book was very amazing and revolutionary.  Lee Jema in his book not only made the extraordinary assertion that distinguishing syndromes is nonsense because the eight main syndromes that are the essence of Chinese medicine (yin, yang, cold, heat, external, internal, excess and deficiency) are already set by the human constitution, but also that illness of the six meridians doesn't come as orderly as prescribed by the famous Chinese doctor Zhang Zhongjing in his book Shanghanlun; that the phases of illness are already set according to the natural born constitution of the individual.

Besides, he evolved his unique organ theory different from that of traditional Chinese medicine.  He dared to criticize the system of Chinese medicine uncritically passed down for millennia, and even shook its roots, indeed a surprising event given the situation of the time, when Chinese science was dominant.

The concept of constitution defined by Lee Jema can be summed up thusly:

Firstly, people are born with different organ structures.
Secondly, these differences affect the form and looks of the body, so a person has a unique figure according to his constitution.
Thirdly, the differences affect the temperament and personality, so a person has a different character.
Fourthly, the differences affect the physiology and pathology of the human body, so the human body manifests different phenomena.
Fifthly, all these differences form a constitution, and therefore a person must use a different cure and method of health care according to his constitution.

However, the constitution theory of Lee Jema until recently wasn't widely accepted even among doctors of Oriental medicine, and only a few scientists studied it and kept it alive. The value of his theory was not much acknowledged, because it was too new a theory to be easily linked to the existing system of Chinese medicine, and too difficult a theory for existing scientists to easily accept.

There was another reason: this scientific system had its own limits.  For example, in order to apply constitution medicine to a patient in clinical practice, it is necessary to first classify the patient's constitution, but the classifying norms explained in his book aren't sufficient to accurately make the classification. Because the medicine used for this constitution medicine has strong characteristics inclined toward one side in contrast to the traditional Chinese medicine, using it on a misclassified patient could bring side effects.  Therefore the scientific evolution was hindered by the reluctance to use it in cases of doubt about the constitution classification.

Lately, though, this constitution medicine is going through some drastic changes.  Because of its amazing and outstanding curative efficacy, it is under constant study by minority scientists and is being completed little by little, giving it its chance to evolve.
In 1965 the outstanding Korean doctor Kwon Dowon published his dissertation, in which he found that the four constitutions discovered by Lee Jema can be divided into eight sub-constitutions, and launched a new kind of acupuncture treatment in addition to the medicine-based treatments presented by Lee Jema.
His constitution acupuncture attracted much attention due to its great and rapid efficacy, and gave people reason to newly acknowledge the theory of constitution medicine on which is based the theory of constitution acupuncture.