The American Society of Anesthesiologists' (ASA) Physical Status Classification was tested for consistency of use by a questionnaire sent to 304 anesthesiologists. They were requested to classify ten hypothetical patients. Two hundred fifty-five (77.3 percent) responded to two mailings. The mean number of patients rated consistently was 5.9. Four patients elicited wide ranges of responses. Age, obesity, previous myocardial infarction, and anemia provoked controversy. There was no significant difference in responses from different regions of the country. Academic anesthesiologists rated a greater number identical than did those in private practice (P < 0.01). There was no difference in ratings between those who used the classification for billing purposes and those who did not. The ASA Physical Status Classification is useful but suffers from a lack of scientific precision.