Correlation between erythrocyte sedimentation rate and C-reactive protein was determined in 239 adult patients in general practice who were suspected of having an infection of the lower respiratory tract. A correlation coefficient of 0.62 (95% CI 0.54-0.69) was found, but there was no association between the results of the two tests when the illness had lasted less than four days. With such a short duration of illness a test for C-reactive protein is to be preferred before erythrocyte sedimentation rate when evaluating lower respiratory tract infections.