Serial C-reactive protein (CRP) and erythrocyte sedimentation rate determinations were compared with clinical course and outcome at 1 to 2 months in 63 children with acute hematogenous osteomyelitis. High CRP values (163 +/- 108 mg/liter) on admission began to descend after the second day of treatment. From the fourth day on higher (P = 0.03 to P = 0.0001) CRP values distinguished a complicated from an uneventful course of acute hematogenous osteomyelitis and the patients symptomatic at follow-up (P = 0.003 to P = 0.0001) from asymptomatic ones. Children who developed extensive radiographic changes had elevated CRP values for a longer time (32 +/- 13 days) than children with typical changes (11 +/- 6 days, P = 0.0001). Erythrocyte sedimentation rates did not identify the type of clinical course but higher values on Days 4 to 7 distinguished children symptomatic at follow-up (P = 0.02) from asymptomatic ones. Monitoring serial CRP values can alert the physician to complications and predict outcome earlier than clinical signs or roentgenograms.