Inhaled pentamidine is used commonly to prevent Pneumocystis carinii pneumonia (PCP) in patients with advanced human immunodeficiency virus infection. Case reports indicate that PCP can recur in patients who receive inhaled pentamidine and that clinical features may be atypical. To determine the magnitude of this problem, we reviewed retrospectively the medical records of patients with proven PCP during a 30-month period at two hospitals. Four (31 percent) of 13 patients with previous PCP who received inhaled pentamidine prophylaxis had recurrent P carinii infection, including one patient with widely metastatic extrapulmonary disease. Chest roentgenographic findings included cavities, pneumothoraces, bilateral and upper lobe interstitial infiltrates, and pleural effusion. False-negative bronchoalveolar lavage and induced sputum examinations were frequent. We conclude that recurrent PCP in patients maintained on a regimen of inhaled pentamidine prophylaxis occurs frequently, causes chest roentgenographic abnormalities other than interstitial infiltrates, and may be difficult to diagnose. Clinicians who choose to use this effective and convenient mode of prophylaxis should be aware of the problems attendant to its use.