Pneumocystis cariniipneumonia was diagnosed in 19 children with cancer. The diagnosis was made in 14 children by pulmonary needle aspiration and demonstration of the organism by methenamine silver nitrate stain. Two children had tension pneumothorax and one had postaspiration hemoptysis following needle aspiration, but there was no mortality from the procedure. Seventeen children were treated with pentamidine isethionate; nine recovered. Toxicity to pentamidine consisted of elevated blood urea nitrogen levels and hypoglycemia.