The value of C-reactive protein (CRP) determinations in the analysis of fever after allogeneic bone marrow transplantation (BMT) was studied prospectively by serial measurements of serum CRP levels during 30 BMT episodes in 28 children and adolescents. The treatments and procedures accompanying BMT did not elicit a significant CRP response. Forty-three febrile episodes were registered and analyzed, without previous knowledge of the results of CRP determinations. The incidence of bacterial infection and acute graft-versus-host disease (GvHD) was low, 8/30 and 5/30, respectively. Raised CRP levels occurred only once in association with GvHD. A CRP level higher than 50 mg/l was not sensitive as an indicator of bacterial infection (4/8). A CRP level below 50 mg/l in the presence of fever, however, excluded bacterial infection with a specificity of 86% and a negative predictive value of 88%. When timed properly and interpreted together with clinical and microbiological findings, CRP measurements can be a valuable aid in the management of fever after BMT, especially as a negative predictor.