Sera collection from 255 clinically diagnosed AIDS and ARC patients were analyzed for antibodies to HIV and HTLV-I by Western blot and particle agglutination methods respectively. Antibodies to HIV were detected in 37.3% of the sera collected as compared to 5.5% for HTLV-I. Most (95%) of the HIV positive sera had dual reactivity to both HIV-I and HIV-2. Antibodies to HTLV-1 were more frequently detected in HIV positive sera (11.58%) than in HIV negative sera (1.88%). Conversely, antibodies to HIV were detected twice as frequently in HTLV-1 positive sera (78.6%) than in HTLV-1 negative sera (34.85%).