Interstitial pneumonitis in immunosuppressed patients demands prompt diagnosis and treatment. In an effort to achieve a simple yet highly accurate method of diagnosis, we have evaluated the usefulness of thoracoscopic examination. Twenty-seven procedures have been performed in 24 patients between the ages of 17 months and 18 years. All patients were immunosuppressed, most for treatment of malignant processes. All procedures have been performed under anesthesia with intravenously administered ketamine, without endotracheal intubation. A definitive diagnosis has been made in every case, with pneumonia due to Pneumocystis carinii being identified in 18 instances. Complications have been minimal and include four minor pneumothoraces, two instances of bleeding, and two instances of prolonged air leak. Mortality attributable to the procedure has been nil, although five patients have died due to their underlying diseases within 30 days of the thoracoscopic procedure. Thoracoscopy has proven to be a rapid and safe technique for providing accurate histologic and bacteriologic diagnoses in these critically ill children.