Thirteen case histories were mailed to 273 general practitioners to study the clinical assessment of the ESR. Participants were asked to state their reference limits for the ESR, as well as action values for the ESR in several clinical situations typical of primary care. The action value should represent the minimal ESR change from a given value necessary to initiate some kind of action towards the patient. The response rate was 76%. In most case histories, half the general practitioners reacted on an ESR change of 10 mm h-1 or less, which is usually due to analytical and biological variation, thus underlining the need for good analytical quality. In general the response was of the same magnitude irrespective of type of case history, i.e. whether the ESR was used in case finding, in diagnosis, or in follow-up. Estimation of the reference limit displayed considerable variation, and knowledge of a previous 'normal' ESR was of minor clinical importance. Furthermore, we found substantial variation regarding the change in ESR necessary to take action in different clinical situations. In principle, for many general practitioners the action value increased as the given ESR increased whereas others reacted on a constant change in ESR, or the change necessary to take action depended on the clinical situation. We conclude that both the different assessments as to the clinical significance of the ESR, and the unawareness of the significance of analytical and biological variation indicate that guidelines for rational use are needed.