Pneumocystis carinii pneumonia (PCP) is the most common opportunistic infection in adults and children infected with the human immunodeficiency virus (HIV). Without prophylaxis, half of all these children will develop PCP at sometime during their illness. The disease is associated with high mortality and a poor prognosis for long-term survival in this patient population. In infants and young children, PCP may be a primary infection, compared with reactivation of a latent infection that is usually the case in older children and adults. Clinical features, radiographic findings and diagnostic strategies are similar in children and adults. Although alternative agents are being investigated, trimethoprimsulfamethoxazole (TMP-SMX) and pentamidine remain the standard therapeutic agents. Insufficient data are available to recommend routine adjunctive corticosteroids in children with acquired immunodeficiency syndrome (AIDS), PCP, and significant respiratory disease. Prophylaxis against PCP occurrence or recurrence is indicated for HIV-infected children and infants under 1 year of age, children with less than 20% T4 helper lymphocytes, those meeting age-related Centers for Disease Control (CDC) guidelines for prophylaxis, and those with a history of suspected or documented PCP. The CDC recommends intermittent TMP-SMX for PCP prophylaxis in children with AIDS.