Epidemiological studies can help to understand the effects of medical treatment of HIV infections. Accordingly, this study was designed to discuss the most important parameters in Frankfurt/Main and other big German cities from 1984 to 2000. The number of HIV tests performed by Frankfurt's Virology has been decreasing continually since 1991. A decrease of new infections in men could be registered, whereas the number of HIV infected women rose. From 1985 to 2000 an annual mean value of 478 HIV infected men and 121 HIV infected women was registered in Frankfurt. The gender proportion was followed up for Frankfurt and Hamburg since 1985, for Berlin, Munich, and Cologne since 1993. All but one city showed a significant decline of infected males, only Berlin did not show any obvious changes in this proportion. Over the last twelve years the average age of men and women tested positive for the first time increased. An obvious shift could be discerned during the last two years concerning the distribution of risk groups. The percentage of HIV infected homosexuals and female i.v. drug addicts sank significantly over the last two years, the number of women infected by heterosexual contacts is still increasing when compared to data compiled from 1988 to 1992, and varies between 44% and 33%. During the same time-span a significant shift in first onset of AIDS-defining illnesses was observed. PCP (pneumocystis carinii pneumonia)--formerly represented with 35.5%--decreased and is now surpassed by tuberculosis with 25.5%. The general gender proportion (3:1) could not be reflected by AIDS-defining diseases of which NHL (non-Hodgkin-lymphoma) seems to have the shortest time-span (6.5 months) between the occurrence of illness and death.