Despite its frequent use and long history, little is known about the diagnostic impact of the measurement of the erythrocyte sedimentation rate (ESR) in general practice. We prospectively followed 362 patients who were seen by their general practitioner (GP) because of a new complaint, for which the GP wanted to know the ESR. The GPs recorded the most probable diagnosis, as well as their clinical judgements in terms of 'severe' and 'not severe', and the expected ESR values, before and after the determination of the ESR in the local hospital laboratory. Having compared the results before ESR determination with the results after, we conclude that ESR is valuable in reassuring the patient and the doctor when 'no pathology' is suspected rather than in confirming the presence of inflammatory diseases and malignancies.