The Family Medicine Group (FMG) model of primary care in Quebec, Canada, was driven by the voluntary implementation of family physicians. Our main objective was to measure the effect of FMG enrolment on avoidable use of the emergency department (ED) by diabetic patients. We also sought to determine if effects differed according to whether patients were infrequent or frequent users of the ED and according to high- versus low-regional levels of enrolment. We used data from provincial health administrative databases to identify the diabetic patient population over the age of 20 years for each fiscal year between 2003-2004 and 2011-2012. We used fixed effects and marginal structural models to estimate the effect of enrolment in FMGs on avoidable use of the ED. Our results indicated that for every 10-percentage point increase in the population enrolled with an FMG in the year prior to an event, there was a 3% reduction in avoidable visits to the ED made by an individual (RR = 0.97; 95% CI = 0.95, 0.99). We found a significant reduction among diabetic patients who had at most 1 visit to the ED per year (RR = 0.97; 95% CI = 0.95, 0.99) and nonsignificant effects among more frequent users. Within low-enrolment regions, a 10-percentage point increase in enrolment in FMG practices at t - 1 led to an 18% decrease in the number of avoidable ED visits (RR = 0.82; 95% CI = 0.78, 0.87). The effect disappeared when the analyses were restricted to the high-enrolment regions (RR = 1.00; 95% CI = 0.92, 1.09). The design and implementation of the incentive to promote team-based practice may not have borne much influence on early adopters who may have been overrepresented by physicians from high-performing practices before the introduction of the reform.