C-reactive protein (CRP) analysis, erythrocyte sedimentation rate and white blood cell count were evaluated as tests in the diagnosis of pneumonia in 84 patients with acute lower respiratory disease. Marked elevated values of CRP were frequently found in pneumonia patients, whereas in most patients with acute asthma, acute exacerbation of chronic obstructive bronchitis and acute bronchitis the values were within the normal range. The combined sensitivities and specificities of the tests were best for CRP, followed by erythrocyte sedimentation rate and white blood cell count.