Eighty-seven inpatients were treated for 93 episodes of Pneumocystis carinii at St Mary's Hospital between January 1989 and December 1990. During this period, 298 patients with the acquired immunodeficiency syndrome (AIDS) were treated at this hospital. Sixteen episodes of pneumothorax occurred and 12 of these, occurring in ten patients, were unrelated to procedure. In six of 12 (50%), the pneumothoraces occurred concurrently with Pneumocystis carinii pneumonia (PCP) and in ten (83%) cases there was a past history of PCP. Bilateral pneumothorax occurred in five cases (42%). In seven (58%) of the cases, patients had been using aerosolized pentamidine as prophylaxis for PCP. This retrospective study confirms the association of pneumothorax with current PCP and also shows an association with previous infection. The use of aerosolized pentamidine was not associated with pneumothorax development. It is important to suspect pneumothorax in a patient with PCP who deteriorates acutely. The high incidence of bilateral pneumothorax means that pleurodesis should be considered early.