From 1994 through 1998, the clinical and demographic features and risk behaviors of 101,945 adolescent and adult patients with acquired immunodeficiency syndrome (AIDS) were reported to the Ministry of Public Health in Thailand. The number of reported cases of AIDS infection increased from 12,005 in 1994 to 24,722 in 1997. Nearly 40% of the cases were reported from the northern provinces, which contained only approximately 20% of the adult population. About 80% of cases were among male patients, and 87% had been acquired via sexual contact. Tuberculosis was the most commonly reported opportunistic infection, occurring in 28.9% of patients; it was more commonly reported among injection drug abusers, especially in Bangkok. Pneumocystis carinii pneumonia and cryptococcal meningitis each occurred in nearly 20% of patients and were more frequently reported in patients with risk factors related to sex than in injection drug abusers. Penicillium marneffei infections were reported in 6.8% of patients from the northern provinces but less frequently elsewhere. These data suggest that AIDS is common in Thailand, and human immunodeficiency virus-infected persons should be given prophylaxis for tuberculosis, fungal infections, and P. carinii pneumonia.