Background: Primary medical care is changing-more female providers, desire for better work-life balance, and increasing availability of walk-in clinics have altered service delivery. There is no uniform physician practice style, and understanding service availability and delivery requires analysis of family physicians' practice patterns, rather than just physician counts., Methods: This paper offers a new approach for describing the practice habits of primary care physicians. We use administrative data to identify activities associated with acting as "most responsible" physicians. We used British Columbia's administrative health care data from 2007/2008 to 2011/2012 to derive information regarding physicians, patients, and service delivery. We developed 5 variables to describe practice style: referrals, oversight, screening, initial prescribing for long-term medications, and repeat visits. Cluster analysis revealed 3 distinct groups of physicians., Results: Only 24% of the primary care physicians were assigned to the high-responsibility group, whereas 36% and 39% were in the low-responsibility and mixed-practice groups, respectively. All cluster variables follow a similar pattern, with the high-responsibility and low-responsibility physicians many multiples apart on the means and the mixed group falling in between. Several forms of sensitivity analysis confirmed the robustness of these results., Conclusions: Physician practice patterns influence the effective supply of primary care. The fact that more than one third of British Columbia physicians are identified as "low responsibility" has implications for the delivery of primary care, both in ensuring that people have access to regular care and in insuring high-quality and comprehensive care.