This study analyzes the use of diagnostic studies by community physicians. Data from billing claims for outpatient visits by Medicaid recipients in three northeastern Pennsylvania counties during a 38-month period were studied, including 55,420 visits to 336 physicians. The use of outpatient laboratory tests and roentgenograms was correlated with the personal characteristics of the prescribing physicians. When case mix was controlled, analysis by specialty showed that internists ordered approximately the same number of procedures as family physicians, and both groups ordered more procedures than general practitioners. The number of years since medical school graduation showed a statistically significant inverse relationship with laboratory and radiology use. Public school graduates ordered more laboratory tests than private school graduates, and foreign medical graduates ordered more roentgenograms than domestic graduates. Other variables, including size and location of practice, and osteopathic or medical training, were not significantly related to patterns of use.