The association of spontaneous pneumothorax and Pneumocystis pneumonia is not frequent. The medical literature reports a ratio between 0 and 5.7%. In 18 patients admitted in our hospital, in the last two years, and diagnosed of AIDS and Pneumocystis pneumonia, we observed two cases of pneumothorax spontaneous. They represent the 11%. In one of them spontaneous pneumothorax was the first manifestation and in the other a complication of the illness. It was not possible to isolate an agent different from Pneumocystis carinii. Considering the lack of predisposition factors and the previous clinical history, we think that Pneumocystis carinii is by itself the agent responsible of this complication. In view of this possibility, spontaneous pneumothorax in a patient who belongs to an AIDS' risk group will alert the clinician to the possibility of infection with Pneumocystis carinii.