This study was designed to explore whether C-reactive protein (CRP) in serum is helpful in assessing the etiologic diagnosis of acute otitis media (AOM) in children. CRP was measured serially by a radial immunodiffusion method in sera from 67 children with AOM and in 67 matched controls, affected by noninfectious neurologic disorders. In the study group 43 (64%) children had a confirmed bacterial AOM and 24 (36%) showed no bacterial growth from middle ear fluid. The upper limit of CRP in controls was 15 mg/liter. Concentrations of CRP in patients with bacterial AOM ranged from less than 6 to 150 mg/liter; in 71% of the cases the value was greater than 15 mg/liter. In the patients with sterile middle ear fluid CRP ranged from less than 6 to 110 mg/liter; in 67% of the cases the level was greater than 15 mg/liter. CRP greater than 15 mg/liter showed sensitivity of 72%, specificity of 33%, predictive value of a positive test of 66% and predictive value of a negative test of 40%, in detecting bacterial AOM. Measurement of CRP should not be used in the decision regarding antimicrobial therapy for AOM in children.