HIV disease progresses from an asymptomatic period of variable duration, through mild symptoms, to severe disease characteristic of cellular immunodeficiency. The rate of progression from infection to severe disease is probably similar world-wide. However, individuals in developing countries have more symptomatic disease, in keeping with the high incidence of morbidity in the general population, and poor survival with advanced disease. The clinical manifestations of severe HIV-related immunosuppression vary with geographical region. Tuberculosis (TB) is the most important severe opportunistic disease in developing countries: the clinical presentation may differ from TB in the immunocompetent. Bacterial infections, particularly due to Streptococcus pneumoniae and non-typhoid Salmonella spp., are also important causes of morbidity and mortality. Fungal diseases such as Pneumocystis carinii pneumonia (PCP), cryptococcosis, histoplasmosis and penicilliosis vary in prevalence in different geographical regions. A high index of suspicion of HIV infection and knowledge of the local spectrum of HIV disease are important for early diagnosis and appropriate management of HIV-related disease.