C-reactive protein (CRP) concentrations were determined in a prospective fashion in 50 children with malignant disease. In 35 children with active and aggressive disease, but without signs of infection, no significant increase in CRP was detected. Neither did aggressive cytostatic therapy (70 courses) in non-infected children result in an increase. In bacteriologically proven, and in clinical sepsis-suspected cases, CRP values increased in all cases to levels above 100 mg/l (normal values less than 5 mg/l). Effective antibiotic therapy resulted in a prompt decline in CRP. Viral infections resulted in a much smaller increase. We conclude that serial measurements of CRP in these immunosuppressed children are of great help in monitoring infections and defining the group that needs antibiotic therapy. The measurement is also a good indicator of the effectiveness of the antibiotic therapy chosen.