OBJECTIVES: Registration of patients with a physician is increasingly being considered across Canada as part of a reform of the primary health care system. This study aims to determine how much 'informal registration' currently exists; that is, to identify the number/proportion of patients who, given existing utilization patterns, already receive the majority of their care from one practice, in order to assess the potential for formal registration. METHODS: Administrative data were used to classify patients (n = 528,905) as being informally registered with a clinic if they received the majority of their care (75% or more of their total ambulatory visits) from the same practice over a three-year period (1994-1996). The proportion and number of informally registered patients were examined. RESULTS: Substantial variability emerged across practices in the proportion of informally registered patients (15-68%) and the number of informally registered patients per physician (544-1378 patients per full-time equivalent physician). Informal registration was higher among rural practices (60%) than among urban practices (38%). CONCLUSIONS: While formal registration of patients with physicians is increasingly being considered in Canada as a means to improve the primary care system, the potential disruption to both patients and physicians in moving towards registration should not be underestimated. The relatively low levels of existing informal registration suggest a need to enhance access by, for example, providing after-hours services.