This study assesses the ability of primary care physicians to diagnose and manage Pneumocystis carinii pneumonia (PCP) in a standardized patient (SP) with unidentified HIV infection. One hundred thirty-four primary care physicians from five Northwest states saw an SP with unidentified HIV infection who presented with symptoms, chest radiograph, and arterial blood gas results classic for PCP. Seventy-seven percent of the physicians included PCP in their differential diagnoses and 71% identified the SP's HIV risk. However, only a minority of the physicians indicated that they would initiate an appropriate diagnostic evaluation or appropriate therapy: 47% ordered a diagnostic test for PCP, 31% initiated an antibiotic appropriate for PCP, and 12% initiated an adequate dose of trimethoprim - sulfamethoxazole. Only 6% of the physicians initiated adjunctive prednisone therapy, even though prednisone was indicated because of the blood gas result. These findings suggest significant delay in diagnosis and treatment had these physicians been treating an actual patient with PCP.