C-reactive protein (CRP) is a sensitive and non-specific inflammatory marker. The serum level of CRP starts to rise 6-12 hours after the start of an inflammatory stimulus. Sequential CRP measurements will have greater diagnostic value than a single measurement, and changes of the CRP values often reflect the clinical course. In use in general practice the diagnostic value of CRP is found to be high in adults with pneumonia, sinusitis and tonsillitis, however it is found to be low regarding otitis and pneumonia in children. As to urinary tract infections and salpingitis the value is still undefined. Measurement of CRP is an important diagnostic test but the analysis should not stand by itself but be evaluated together with the patient's history and clinical examination.