C reactive protein (CRP) was measured serially in 29 patients with infective endocarditis. Twenty one patients were initially treated with antimicrobial drugs. In 13, serial measurement of CRP concentrations showed a progressive return to normal (less than 10 mg/l), which correlated with a satisfactory recovery. Of the remainder (eight patients), five had persistently high concentrations of CRP, indicating a failure to respond to antimicrobial treatment alone. Two of these five patients died and three underwent valve replacement. Of 11 patients treated with antibiotics and valve replacement, CRP concentrations returned to normal in nine. Two patients had infective complications and the CRP concentration did not return to normal. A transient rise in CRP concentration during an otherwise uneventful fall to normal was a sign of allergic reaction in two and of intercurrent infection in three more patients. Serial measurements of CRP concentrations in patients with infective endocarditis may be useful to monitor treatment and also to detect other infections and complications.