We prospectively evaluated the accuracy with which clerical and laboratory staff carried out physicians' written orders for thyroid function testing in 181 patients at two institutions. In 54% of the patients studied, support staff were found to have added or deleted one or more tests from the original written orders. When the ordering physicians were asked to identify the clinical indications for the tests they had ordered, 37% of their orders either lacked an appropriate test or included an unnecessary test request. In contrast, after clerical and laboratory staff had changed orders, only 25% of patients had inappropriate tests performed or necessary tests omitted. Clerical staff using computer-based ordering menus significantly improved the appropriateness of physicians' orders compared with clerks who lacked such guidance. Laboratory technologists who used informal, knowledge-based rules also tended to improve the appropriateness of physicians' orders. We conclude that a substantial proportion of physicians' orders for diagnostic tests may be modified during the test requisitioning process. The use of testing regimens by ward clerks and laboratory technologists may explain their ability to improve on physicians' orders.