Genetic testing for inherited germ-line mutations associated with cancer susceptibility is an emerging technology in medical practice. Limited information is currently available about physician use of cancer susceptibility tests (CSTs). In 1999-2000, a nationally representative survey was conducted to estimate prevalence of CST use by United States physicians and assess demographic, training, practice setting, and practice patterns associated with use. A stratified random sample of clinicians in eight specialties was selected from a file of all licensed physicians. In total, 1251 physicians, including 820 in primary care and 431 in tertiary care, responded to a 15-min questionnaire by mail, telephone, fax, or Internet (response rate = 71.0%). In the previous 12 months, 31.2% [95% confidence interval (CI), 28.5-33.9] overall, including 30.6% (95% CI, 27.5-33.7) in primary care and 33.4% (95% CI, 27.9-38.9) in tertiary care, had ordered CSTs or referred patients elsewhere for risk assessment or testing. More physicians referred patients elsewhere [26.7% (95% CI, 24.2-29.2)] than directly ordered tests [7.9% (95% CI, 6.3-9.5)]. Factors associated with ordering or referring included practice location in the Northeast [odds ratio (OR), 2.30; 95% CI, 1.46-3.63%], feeling qualified to recommend CSTs (OR, 1.96; 95% CI = 1.41-2.72), receiving CST advertising materials (OR, 1.97; 95% CI, 1.40-2.78%), and most notably, having patients who asked whether they can or should get tested (OR, 5.52; 95% CI, 3.97-7.67%). Lower CST use was associated with not knowing if there were local testing and counseling facilities (OR, 0.39; 95% CI, 0.23-0.66%). These findings underscore the importance of establishing effective clinical approaches to test use and promoting physician education to facilitate communication with patients about cancer genetics.