To better understand the reasons why some fee-for-service physicians have high billing levels, the authors compared the practice and demographic characteristics of general practitioners and family physicians (GP/FPs) who submitted over $400,000 in annual Ontario Health Insurance Plan (OHIP) fee-for-service claims in 1994/95 with those GP/FPs who billed between $35,000 and $400,000. They used multivariate logistic regression to determine factors independently associated with high billing status. A total of 219 GP/FPs (2.5% of the GP/FPs in Ontario) billed over $400,000 in 1994/95. Of these, 14 had billing patterns similar to those of specialists, and 27 billed predominantly for diagnostic and therapeutic procedures (particularly physiotherapy). The remaining 178 (81.3%) billed for a mix of services similar to that of other GP/FPs but on average had 2.6 times the volume of patient assessments and a greater share of their total billings derived from diagnostic and therapeutic procedures (9.1% v. 5.6%). Multivariate analysis indicated that these high-volume GP/FPs were less likely than GP/FPs who billed between $35,000 and $400,000 to be 60 years of age or older (odds ratio [OR] 0.09, p < 0.05) and female (OR 0.21) and were more likely to be foreign graduates (OR 1.85) and practising in a region with low physician suly (OR 0.45 for each increase of 1 physician per 1000 population). Metropolitan Toronto was an outlier to the latter relation and was more likely to have high-volume GP/FPs (OR 16.89). High-billing GP/FPs attained their high billing levels by maintaining large numbers of patient visits and by performing procedures. Further research is needed to determine the time spent per patient and the quality of care delivered by these physicians as well as the aropriateness of the procedures that they perform.