Variations in physician practice patterns have important implications for quality and cost. The purpose of this article is to explain variation in physicians' practice patterns in terms of physician personal characteristics, practice setting, patient population, and managed care involvement. Data on 2,455 primary care physicians were derived from the Community Tracking Study Physician Survey (1996-1997). Factor scores were determined based on responses to three clinical scenarios that represent discretionary medical decisions. These scenarios include a specialist referral for benign prostatic hyperplasia, prescription drugs for elevated cholesterol, and an office visit for vaginal discharge. Physician age, being a foreign medical school graduate, being a solo practitioner, and having a larger proportion of Medicaid patients were all associated with higher factor scores, a greater likelihood of ordering a service. Being board certified was associated with lower factor scores. Managed care involvement was not a significant predictor of factor scores.