The acute phase C-reactive protein (CRP) was measured in serum of HIV-infected patients suffering from Pneumocystis carinii pneumonia (PCP) (32 patients), bacterial pneumonia (10 patients), and in 19 immunocompetent patients with bacterial pneumonia. The HIV-infected patients with bacterial pneumonia had a significantly lower CRP level than the immunocompetent patients (50% versus 95% had an s-CRP level > 80 mg/l). No significant difference was found in the CRP response to P. carinii or bacteria in HIV-infected patients with pneumonia due to these microorganisms (20% versus 50% had s-CRP > 80 mg/l). In the group of PCP patients, a significantly lower CRP level was found in those with CD4 positive lymphocyte counts below 50 x 10(6)/l. There was no correlation between the CRP response and the severity of the PCP as estimated by the degree of hypoxia. We conclude that the CRP level cannot be used to discriminate between PCP and bacterial pneumonia in HIV-infected patients.