The usefulness in the diagnosis of pneumonia of temperature and the laboratory tests: erythrocyte sedimentation rate (ESR), leucocyte count, and C-reactive protein (CRP) was evaluated against a radiographic reference standard in 402 adult patients with respiratory tract infection in general practice. Radiographic pneumonia was diagnosed in 20 patients. CRP and ESR were the most useful tests. CRP greater than 50 mg/l had lower sensitivity and likelihood ratio (LR), 0.50 and 4.8, respectively, compared with previous studies of selected patient populations. Among patients whose duration of illness exceeded six days the corresponding LR was 11.3, due to a higher specificity in this subgroup of patients. ESR and oral temperature were also more useful in this subgroup than in patients with a shorter duration of illness. A highly significant diagnostic contribution of adding ESR and CRP to history and physical examination, particularly when the illness had lasted one week or more, was demonstrated by logistic regression.