The prospective Swiss HIV Cohort Study (SHCS) documents since 1988 epidemiological, clinical and laboratory data of HIV-infected participants. Until the end of 1997, 9447 patients with a total follow-up of 25,584 patient-years have been included. The proportion of women is 27.3% and is rising. The proportion of heterosexually infected patients has been rising since 1992, this group was with 35.6% in 1997 the most frequent transmission category within the newly registered participants. The SHCS is representative for Switzerland since 73% of the 5532 nationwide registered AIDS cases are included in the study. Recently, the SHCS data could demonstrate an impressive reduction in morbidity and mortality due to the improvement in therapy and prophylaxis. The SHCS also serves as a basis for large multicenter studies that aim to evaluate new prophylactic treatments or new antiretroviral strategies. By introducing uniform guidelines for the diagnosis and the treatment of HIV infection, the SHCS has contributed significantly to the high quality of patient care, a goal that must be maintained and further improved in the future. Also important is the recognition of acute HIV infections because early treatment of such patients holds the promise to substantially improve the clinical course.