OBJECTIVE: To examine the importance of cytomegalovirus (CMV) in bronchoalveolar lavage (BAL) fluid of patients with HIV-associated Pneumocystis carinii pneumonia (PCP) treated with adjunctive corticosteroids (CS). DESIGN: Analysis of clinical data during a 5-year period. SETTING: Department of infectious diseases where clinical and paraclinical data on patients suspected of having PCP have been sampled prospectively. PATIENTS: 148 consecutive patients with a first episode of PCP in a 5-year period. MAIN OUTCOME MEASURE: Vital status 3 months after diagnosis of PCP. RESULTS: Patients with PCP treated with adjunctive CS who had CMV cultured from BAL fluid had a two times higher mortality within 3 months from bronchoscopy than others (p = 0.08). This difference could not be explained by differences in CD4 count, PO2 or PCO2 at time of bronchoscopy. CONCLUSION: With the accepted usage of adjunctive CS in severe PCP, the role of CMV as a pulmonary copathogen may have changed. Active CMV infection may be an important cause of failing treatment of severe PCP in those treated with adjunctive CS.