To evaluate the aetiology of febrile episodes and to rationalize our politics with antibiotics, C-reactive protein (CRP) was determined immunoturbidimetrically in 20 consecutive neutropenic adults with acute leukemia. They had 35 febrile episodes, 89% of which were infectious. Twenty per cent of infections were fungal. A similar CRP response was seen both in bacterial and in fungal infections. In 84% of infections the peak value for CRP rose greater than 100 mg/l. Thirty-five apyrexial patients with acute leukaemic and 20 healthy adults served as controls. Their CRP was less than 10 mg/l in 87%. CRP proved most valuable in the follow-up of infections, in the detection of infectious complications and in the detection of possible invasive fungal infections. Although relapse itself did not effect on CRP levels, extramedullary bone infiltration in two of our patients resulted in increased CRP production, which normalized with cytostatics only.