One hundred forty-six physicians responded to two written patient vignettes by recording problem lists, diagnostic impressions, and the tests and procedures they would have ordered after the initial encounter with the hypothetical patient described in each vignette. Wide variations within medical specialty were found in the number and types of tests or procedures ordered and their costs. We conclude that little consensus exists among physicians as to types of tests and procedures to order, and that private habits or rituals may account for much of the variation in ordering patterns. These findings point to the need for intensive study of how physicians respond to clinical ambiguity as well as how test-ordering habits and rituals arise and persist in different practice settings even within the same specialty. The health care cost implications are substantial.