We describe three patients with Pneumocystis carinii pneumonia as the initial presentation of severe combined immunodeficiency disease. The pneumonia in the first patient was treated successfully with trimethoprim/sulphamethoxazole (Tmp/Smz). The second patient died despite therapy with Tmp/Smz and pentamidine. The third patient failed to respond to therapy with Tmp/Smz and pentamidine. He was subsequently treated with trimetrexate and leucovorin. Treatment with the new folic acid antagonist trimetrexate resulted in complete recovery. The case histories of these children serve to illustrate the clinical symptoms and new therapeutic modalities of P. carinii pneumonia in patients with immunodeficiency disease.