The Italian Cooperative Group on AIDS & Tumors (GICAT) is collecting data on the epidemiological and clinical features of Kaposi's sarcoma (KS) in Italy, a country where intravenous drug users (IVDUs) comprise the largest group affected by AIDS. As of December 31st 1989, among 5317 cases of CDC-defined AIDS reported to the National Registry of AIDS, there were 419 (8%) cases of KS. Twenty-five percent of all of the homosexual men with CDC-defined AIDS had KS, while only 3% of the IVDU patients with CDC-defined AIDS had KS. The first case of KS was observed in 1982, 2 cases in 1983, 7 cases in 1984, 17 cases in 1985, 55 cases in 1986, 89 cases in 1987, 120 cases in 1988 and 128 cases in 1989. Of the 226 cases of KS reported to GICAT, 60% were in homosexual men, 21% in IVDUs, and 9% in heterosexuals. An increased number of KS has been observed since 1982, although the number of new cases in the past year seems to have been stable, possibly due to the usual delay in notification. There is, however, an actual decrease of the percentage of KS among AIDS patients, from 18% in 1984 to 6% in 1989. Almost half of the patients had had a diagnosis of AIDS prior to the appearance of KS, but in 25% of the patients KS was the first AIDS manifestation. Almost all patients had skin involvement either alone or in association with other sites, whereas very few patients had only lymph node involvement. Only treatment with alpha-2 interferon was able to obtain complete remission in 9% of evaluable patients.(ABSTRACT TRUNCATED AT 250 WORDS)