Because of lower respiratory infection that was treated with antibiotics on the suspicion of pneumonia, 71 patients aged 15 years or more were referred to the study by general practitioners. Using a positive chest X-ray as a "gold standard", 15% had pneumonia. The diagnostic value of variables from history, physical examination and blood tests was evaluated by calculating the likelihood ratio (LR). A duration of illness less than 24 hours before consulting the general practitioner was the variable from the history with the highest LR, 13.5. The white blood cell count and particularly the C-reactive protein analysis had a high diagnostic value, CRP greater than 50 mg/l had an LR of 37. In this selected material pulmonary symptoms and lung findings were of minor value in differentiating patients with and without pneumonia, with no LR exceeding 2.3. This can be explained to some extent by selection bias.