Isocyanates are very useful chemicals, and these are important for our daily life. Various products, especially urethane resin, are made from these chemicals. There is no substitute for isocyanates at the present. Isocyanates are patent agents to provoke immunological responses. This article deals with the case reports of hypersensitivity pneumonitis (HP) induced by isocyanates in Japan. It is said that HP is one of the rare diseases induced by isocyanate with very low frequency so far in the world. Certainly there are rare cases of isocyanate-HP also in Japan, but these are the cases only in large enterprises that pay attention to their working places. In Japan, there are several cases of isocyanate-induced HP, which happened in minor enterprises such as those with less than 50 workers. In clinical appearance, there are few conflicts with Western studies. The best choice for sensitized people is isolation from working places. It is important for the workers to understand the diseases. In these days, several tests for diagnosis are developed. It is indispensable to put these tests for prevention of these diseases and improvement of their working places.