Understanding the reasons physicians order diagnostic tests may be useful in designing strategies to promote cost-effective practice and helpful in developing appropriate endpoints for evaluations of new diagnostic technologies. Testing by 27 private office physicians for 324 hypertensive patients was examined and the doctors were surveyed for their beliefs about appropriate levels of use for common tests, how they compared with other doctors in frequency of use, and their reasons for ordering tests. Beliefs about appropriate levels of testing were correlated with actual use (0.45 to 0.63, Pearson). Usage also correlated with beliefs about relative ordering patterns. Annual testing charges incurred by physicians who believed they ordered tests "more frequently" than average were higher than those who believed they ordered tests "less frequently" than average ($122 +/- $46 versus $51 +/- $15, p less than 0.05). Reasons for ordering tests were (in order of importance): establishing a baseline, assessing prognosis, reassuring patients, and helping with treatment decisions. These results suggest that "high usage" physicians may know who they are; and therefore strategies of feedback to modify behavior should be designed to do more than educate physicians about their own utilization. Second, evaluations of technology must consider more than contributions to immediate treatment decisions.