Little is known about the effect of human immunodeficiency virus (HIV) infection on the Central American healthcare system. We describe HIV-related admissions in a Guatemalan medical service. The study was conducted at Guatemala City's largest public hospital. Data were derived from standardized data collection sheets maintained by the HIV testing service and by HIV clinic physicians. Data were collected for 295 medicine admissions of 257 HIV-infected adults during an 18-month period in 1999 and 2000; 30% of the patients were women. Average age was 33 years. Only 12.5% of the patients had been diagnosed with HIV infection prior to 1999 and nearly all had symptomatic AIDS. 60.3% of the patients were diagnosed with HIV infection during their hospitalization. The most common discharge diagnoses were tuberculosis (13.9%), toxoplasmosis, diarrhoea, candida and other fungal infections, and meningitis. Mean length of stay for HIV-positive patients was 17 days. 23.7% of the patients died during their hospitalization; this was double the mortality of non-HIV patients. HIV-infected patients represented 5.8% of the total admissions of the general medical wards. In a country where HIV prevalence is thought to be less than 1%, AIDS is now responsible for over 5% of admissions to a large medical service at a cost of $500,000 per year. These findings underline the importance of HIV infection in Central America and demonstrate the utility of tracking hospital admission data as a method of surveillance.