Serum C-reactive protein (CRP) levels were studied in 79 children with acute otitis media (AOM), aged from 4 months to 5 years. The CRP was less than 10 mg/l in 27 children, greater than or equal to 20 mg/l in 34, and greater than or equal to 40 mg/l in 17 children, 25 of the 41 attacks caused by S. pneumoniae or H. influenzae showed a CRP of greater than or equal to 20 mg/l and 15 CRP greater than or equal to 40 mg/l, in 38 cases without major otitis pathogens, the respective figures were 9 (p less than 0.01) and 2 (p less than 0.001). Although statistically significant correlations between otitis-related clinical parameters and CRP levels were rare, there was a tendency toward higher CRP values among those with a more severe clinical picture. All five attacks with CRP greater than or equal to 100 mg/l were bilateral, caused by major pathogens, and preceded by a respiratory infection. They also tended to have high fever and a large amount of fluid in myringotomy. However, even in these the general course of AOM and other morbidity was not different from the others.