Serum concentrations of carcinoembryonic antigen (CEA) were measured in 43 consecutive patients with HIV-related Pneumocystis carinii pneumonia (PCP). The subjects were divided into 2 groups according to the severity of the PCP:PaO2 in ambient air (AA) less than or equal to 50 mmHg on admission (n = 22, group 1) and PaO2 greater than 50 mmHg (n = 21, group 2). In addition, 57 HIV patients with either non-PCP pulmonary diseases (n = 34, group 3) or extrapulmonary disease (n = 23, group 4) were studied. Mean CEA levels (ng/ml) were 13 +/- 10 in group 1 and 4.9 +/- 5.5 in group 2 (p less than 0.001). The corresponding values in groups 3 and 4 were much lower (2.7 +/- 1.8 and 2.4 +/- 1.8, respectively). In group 1, mean initial CEA levels were higher (p less than 0.001) in the patients who died (n = 6; 23.5 +/- 11) than in the survivors (n = 16; 8.9 +/- 7), although the initial mean PaO2 were identical (39 +/- 7 and 39 +/- 8 mmHg, respectively) and the initial mean LDH levels were not significantly different (1544 +/- 530 and 1200 +/- 457 IU/l). CEA levels fell during specific anti-PCP therapy associated with corticosteroids but returned to normal only in the survivors. We conclude that CEA levels are increased in patients with PCP and acute respiratory distress. Among the patients with PaO2 levels of less than or equal to 50 mmHg before treatment, only high levels of CEA (greater than 20 ng/ml) were associated with a fatal outcome, regardless of anti-PCP therapy associated with corticosteroids.(ABSTRACT TRUNCATED AT 250 WORDS)