It is controversial whether physicians’ interactions with drug companies affect their behavior. To test the null hypothesis, that such interactions are not associated with physician behavior, we studied one behavior: requesting that a drug be added to a hospital formulary. —Nested case-control study. —University hospital. —Full-time attending physicians. Case physicians were all 40 physicians who requested a formulary addition from January 1989 through October 1990. Control physicians were 80 randomly selected physicians who had not made requests. —Physician interactions with drug companies, as determined by survey of physicians (response rate, 88% [105/120]). —Physicians who had requested that drugs be added to the formulary interacted with drug companies more of ten than other physicians; for example, they were more likely to have accepted money from companies to attend or speak at educational symposia or to perform research (odds ratio [OR], 5.1; 95% confidence interval [CI], 2.0 to 13.2). Furthermore, physicians were more likely than other physicians to have requested that drugs manufactured by specific companies be added to the formulary if they had met with pharmaceutical representatives from those companies (OR, 13.2; 95% CI, 4.8 to 36.3) or had accepted money from those companies (OR, 19.2; 95% CI, 2.3 to 156.9). These associations were consistent in multivariable analyses controlling for potentially confounding factors. Moreover, physicians were more likely to have requested formulary additions made by the companies whose pharmaceutical representatives they had met (OR, 4.9; 95% CI, 3.2 to 7.4) or from whom they had accepted money (OR, 1.7; 95% CI, 1.0 to 2.7) than they were to have requested drugs made by other companies. —Requests by physicians that drugs be added to a hospital formulary were strongly and specifically associated with the physicians’ interactions with the companies manufacturing the drugs. (JAMA. 1994;271:684-689). © 1994, American Medical Association. All rights reserved.