We used continuous positive airway pressure (CPAP) by face mask to treat 18 AIDS patients with Pneumocystis carinii pneumonia (PCP) who were in hypoxic respiratory failure. Candidates for mask CPAP were conscious, not hypercarbic, and able to protect their airway on ICU admission. Treatment was effective and well tolerated. Mean Po2 rose from 62 to 158 torr, respiratory rate decreased from 51 to 32 breath/min, and Pco2 was unchanged. Mean duration of treatment was 4.5 days. Only one patient developed a pneumothorax; there were no other major complications. Hospital mortality was 55%. CPAP by face mask allows speech and permits discussion of therapeutic limits. We present our protocol for using CPAP by face mask and conclude that CPAP is effective supportive therapy in hypoxic respiratory failure complicating PCP and AIDS.