The goals of our work were to study prospectively the possibility of differentiating between infections and neoplastic fever in adult cancer patients on admission, by means of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) or of follow-up CRP values. Patients and methods were as follows: the final infection group consisted of 56 patients and the noninfection group of 10 patients with neoplastic fever; CRP was measured on days 0, 3 and 5 and ESR at entry. The main results showed that the median CRP did not differ between the groups (91 mg/l vs 102 mg/l) on entry, while the ESR level was higher in the neoplastic fever group (50 mm/H vs 89 mm/H, P = 0.023). On admission, both markers had low area under receiver operating characteristic curves for the demonstration of infection (CRP 0.42; ESR 0.27). The CRP level dropped significantly in the infection group within 5 days (P = 0.009). We conclude that neither of the markers was useful in differentiating between infections and neoplastic fever on admission, but that the follow-up CRP values were advantageous in this respect.