We examined the value of serum C-reactive protein (CRP) in febrile children without an apparent focus of infection, (i) as a tool to differentiate bacteraemia and bacterial infection from a non-bacterial illness (NBI), and (ii) as an indicator of recovery or complications. Included in the study were 100 children up to the age of 3 years with a temperature of > or = 38.5 degrees C, without an apparent focus. The serum CRP concentration was measured on days 1, 3 and 5 of evaluation and correlated with the final diagnosis and outcome. The serum CRP was 40 mg/l and above in 95% of patients (18/19) with bacteraemia and also in seven of the eight with purulent meningitis, while it was < 40 mg/l in 84% of patients (52/62) with NBI (mean (SD) 22 (28.6) mg/l). The mean serum CRP concentration among six children with a culture-positive urinary tract infection (16.3 (8.3) mg/l) and five with otitis media (9 (5.7) mg/l) was similar to those with NBI. The sensitivity of serum CRP > or = 40 mg/l for diagnosis of bacteraemia was 95% and the positive predictive value 67%. On serial monitoring, a fall in the CRP concentration was a sensitive indicator of recovery from infection and provided the earliest clue to therapeutic response long before a fall in temperature.