This study aimed to examine trends in incidence, geographical distribution, and survival of classic and AIDS-related Kaposi's sarcoma (KS) in the general US population using Surveillance, Epidemiology, and End Results (SEER) tumour registries with 12 066 patients diagnosed with KS between 1975 and 2005. Although the age-adjusted standardized incidence ratio (SIR) of AIDS-related KS (1·9) during 1980-2005 was not significantly higher than that of classic KS (1·4) during 1975-2005 (P = 0·78), the trends in annual SIR rates revealed distinct patterns. While the SIR for AIDS-related KS declined across all registries from the early 1990s (4·6) to late-1990s (0·3) (P = 0·05), the SIR of classic KS remained relatively steady (1·7). In both forms the SIR of KS was highest in metropolitan areas. The 5-year survival rates for patients with AIDS-related KS improved from 12·1% (1980-1995) to 54% (1996-2005) (P = 0·05). Survival rates for patients with classic KS remained stable, ranging from 75·7% to 88·6% during the 30-year period. These results may reflect improved HIV treatment.