The acute phase response induced by 19 episodes of Pneumocystis carinii pneumonia (PCP) was analysed in a retrospective study. There were 9 men and 1 woman with HIV, aged 25-45 years (mean 37.2) and 4 men and 4 women with other immunodeficiencies, aged 18-35 years (mean 26.2). The outcome of these two groups with PCP did not differ: in the HIV group 4 died and in the non-HIV group 2 died. In the HIV group, peak serum C-reactive protein (S-CRP) ranged 41-228 mg/l, mean 126 (77.3), and in the non-HIV group 19-290 mg/l, mean 105 (89.1) (NS). The patients with a fatal outcome had higher mean peak S-CRP: 186 (73.8) mg/l versus 85.3 (63.5) mg/l in the survivors (p = 0.007). The HIV infection itself did not increase the S-CRP concentration, which was normal before or after PCP in all 7 survivors of the 11 episodes. Thus, PCP induces a clear-cut S-CRP response, which may be used for monitoring the treatment and evaluating the prognosis of the patient.