Western medicine was introduced to Japan by the Portuguese around 1590, a few decades after their first landing in Japan in 1543. Additional entry of Western medicine came via Spain during the early 1600s, but this influence of Portugal and Spain was limited to the coastal regions, such as Kyushu (one of the Japanese islands).

The Japanese leadership adopted a policy of national isolation soon after the Portuguese and Spanish incursions, allowing trade and interchanges only with the Dutch (among Western powers) starting in 1639, which brought entry of Dutch physicians and medical works.

This foreign medicine took on significance only after Japanese scholars studied it and wrote about it from their perspective. For example, in 1774, Genpaku Sugita published Kaitai-shinsho (The New Book of Anatomy), which greatly enhanced the reputation of Western medicine. Another scholar of Western medicine was Gentaku Otsuki, who pointed out that the two systems of medicine could be complementary, each overcoming the inadequacies of the other. During this time, traditional practitioners began to develop specialization like that found in Western medicine; for example, Genetsu Kagawa (1700-1777; see Figure 3) focused on obstetrics and influenced the field for generations afterward. Seishu Hanaoka (1760-1835; see Figure 4) actively pursued integration of Kampo and Western medicine.

The biggest boost to Western medicine in Japan came with the introduction of smallpox vaccination in 1849, fifty years after it had been developed in Europe. Kampo had no comparable disease prevention techniques and its reputation suffered under the light of the successful vaccination campaign. Kampo then went into a period of severe decline from 1868 to 1902 as the result of efforts by the Meiji government. The government based its actions on the concept that Japan was too weak relative to the Western nations, and took drastic steps to build up the military and to strengthen the internal affairs. As part of this, Western medicine was accepted as the most desirable method of therapy and was fully endorsed by the government, with efforts made to suppress the traditional system as being outmoded and contributing to the weakness of Japan.

New medical examinations (starting in 1875) focused on chemistry, anatomy, physiology, pathology, pharmaceutics, internal medicine, and surgery, and ignored traditional medicine dogma, herbs, acupuncture, and other aspects of the ancient medicine. In 1883, the government required passing of the new examination even for those who wished to practice Kampo. The medical studies required for this exam were so intensive as to virtually preclude the opportunity to also study Kampo, even if one wished to do so in this climate.