The authors retrospectively reviewed the medical records of HIV/AIDS patients who were admitted to the medical service, Siriraj Hospital from January 1, 2002 through December 31, 2002. Demographics, CD4 lymphocyte counts, discharge diagnoses, the incidence of Pneumocystis carinii pneumonia (PCP), cerebral toxoplasmosis and cryptococcosis in patients who received and did not receive appropriate chemoprophylaxis against those opportunistic infections when indicated, and outcome of the patients were collected. Three hundred medical records of 286 HIV/AIDS patients were available for review. One hundred and seventy two patients (60.1%) were male. Mean age of the patients was 36.8 +/- 9.91 years (range 14-74). The mean CD4 lymphocyte count that was determined in 165 patients was 74.7 +/- 134.21 cells/mm3 (range 0-894). Of the 300 admissions, 36 per cent were newly diagnosed HIV infection. Only 23 (7.7%) patients had received antiretroviral drugs at the time of hospitalization. The leading HIV-related diseases were tuberculosis (29.3%), Pneumocystis carinii pneumonia (18.7%), and cryptococcosis (15.7%). The rest of them included cytomegalovirus diseases (6.3%), lymphoma (6.3%), Salmonella bacteremia (6%), cerebral toxoplasmosis (5.7%), cryptosporidiosis (5.3%), disseminated Mycobacterium avium complex infection (1.0%), extrapulmonary histoplasmosis (1.0%), Candida esophagitis (1.0%), progressive multifocal leukoencephalopathy (1.0%), and rhodococcosis (0.7%). Among those for whom HIV infection was established and chemoprophylaxis for PCP, cerebral toxoplasmosis and cryptococcosis were indicated, 9.8 per cent vs 28.2 per cent, 3.6 per cent vs 5.1 per cent, and 10 per cent vs 15.2 per cent of whom received and did not receive the appropriate chemoprophylaxis developed PCP, cerebral toxoplasmosis and cryptococcosis respectively. One hundred and ninety (63.3%) patients were alive at discharge, 84 (28.0%) had died, 21 (7%) were referred to other hospitals, and 5 (1.7%) left hospital against medical advice. The mortality rate in newly diagnosed HIV and in known HIV without antiretroviral treatment were comparable but much lower in known HIV-infected patients who received antiretroviral therapy. Secondary prevention by detection of HIV-infected patients while they are asymptomatic and providing them with appropriate chemoprophylaxis against specific opportunistic infections as well as appropriate antiretroviral treatment would decrease morbidity, mortality, and improve the quality of life of HIV-infected patients in Thailand.