University hospital Medical Information Network (UMIN) was established in 1989 as a collaborative project of national university hospitals in Japan funded by the Ministry of Education, Science, Sports, and Culture, and it started its Internet-based service for medical professionals in 1995. Since then, many services including those for academic societies and research groups have been added. Currently, due to batch registration of the members of large academic societies, the number of registered users and Web accesses are rapidly increasing (about 87,000 registered users and 4,500,000 page views in November, 2000). More than one hundred homepages of academic societies and research groups (including forty- two member-only ones) and one thousand mailing lists are operated. More than one hundred thirty academic societies collect abstracts using the UMIN server. All the Internet-based clinical and epidemiological research projects in Japan are now under way using UMIN. The characteristics of UMIN are its large variety of services and large number of user accounts of medical professionals, which are beneficial to both users and information service providers. UMIN is now important for some medical specialties and will be so for further ones in the future. UMIN is, in fact, evolving into a collaborative project of the Japanese medical community and is considered as its key information infrastructure.