To determine the usefulness of the quantitative serum C-reactive protein (CRP) concentration as a tool in differentiating acute pneumonia from other acute lower respiratory tract infection (ALRI), and in assessing response to therapy serum CRP concentrations were measured in 30 children with pneumonia, 30 with acute bronchitis and 5 with bronchiolitis. All the pneumonia patients had a serum CRP level above 83 mg/l (mean 157; SD +/- 47 mg/l) whereas none of the acute bronchitis (mean 7.8; SD +/- 5.7 mg/l) or bronchiolitis patients and normal children (mean +/- 3.5 SD +/- 2.7 mg/l) had a level above 35 mg/l. The sensitivity and positive predictive value of CRP level greater than 35 mg/l for diagnosis of pneumonia was 100%. On serial monitoring a fall in CRP concentration also provided the earliest clue to therapeutic response, much before a fall in temperature, respiratory rate or ESR.