Despite high seroprevalence there are few recent studies of the effect of human immunodeficiency virus (HIV) on hospitals in sub-Saharan Africa. We examined 1226 consecutive patients admitted to medical and surgical wards in Blantyre, Malawi during two 2-week periods in October 1999 and January 2000: 70% of medical patients were HIV-positive and 45% had acquired immune deficiency syndrome (AIDS); 36% of surgical patients were HIV-positive and 8% had AIDS. Seroprevalence rose to a peak among 30-40 year olds; 91% of medical, 56% of surgical and 80% of all patients in this age group were HIV-positive. Seropositive women were younger than seropositive men (median age 29 vs. 35 years, P < 0.0001). Symptoms strongly indicative of HIV were history of shingles, chronic diarrhoea or fever or cough, history of tuberculosis (TB), weight loss and persistent itchy rash (adjusted odds ratios [AORs] all > 5). Clinical signs strongly indicative of HIV were oral hairy leukoplakia, shingles scar, Kaposi's sarcoma, oral thrush and hair loss (AORs all > 10). Of surgical patients with 'deep infections' (breast abscess, pyomyositis, osteomyelitis, septic arthritis and multiple abscesses), 52% were HIV-positive (OR compared with other surgical patients = 2.4). Severe bacterial infections, TB and AIDS caused 68% of deaths. HIV dominates adult medicine, is a major part of adult surgery, is the main cause of death in hospital and affects the economically active age group of the population.