Pulmonary infection with cytomegalovirus (CMV) is a well recognised complication of AIDS. It is often possible to detect CMV-infected cells in bronchoalveolar lavage (BAL) specimens with monoclonal antibodies, but the clinical significance of their presence remains unclear. To investigate this, 24 AIDS patients were tested for CMV antigenaemia and viraemia, in addition to CMV detection in BAL. CMV was detected in the BAL of nine patients (38%), five with clinical and laboratory evidence of pulmonary infection and four without pulmonary involvement. Blood samples positive for CMV antigen were observed in two patients with CMV-positive BAL specimens and, in both cases, antigenaemia resolved without therapy. No case of viraemia was detected. Pneumocystis carinii was detected concomitantly with CMV in the BAL of four of the patients with pulmonary involvement and in one without signs of pulmonary infection. These data suggest that CMV-positive BAL results are of limited significance in the diagnosis of CMV pneumonia in AIDS patients, unless associated with high levels of antigenaemia or viraemia and compatible clinical symptoms.