The acute-phase protein C-reactive protein (CRP) is a sensitive marker of inflammation and tissue damage. We measured CRP in 109 HIV-1 antibody-positive patients admitted to hospital for investigation. In 67 patients with intercurrent infection (of whom 27 were afebrile at presentation) CRP levels were 2.2-483.5 mg/ dL (normal value in the general population <3mg/dL) and in 42 patients with alternative non-infection diagnoses CRP levels were 0.5-108.6 (median = 5.9) mg/ dL. Whereas in those with infections elevated CRP levels fell in response to specific therapy, values remained abnormal in those with non-infection diagnoses. CRP appears useful for diagnosis and monitoring of intercurrent infection in HIV-1 antibody-positive patients. In HIV-1 antibody-positive patients without intercurrent infection, CRP values higher than in the general population possibly reflect a sustained acute-phase response as a consequence of HIV infection per se.