The ratio of patients to physicians has long been used as a tool for measuring and planning healthcare resources in Canada. Some current changes in primary care, such as enrolment of patients with physicians, make this ratio easier to calculate, while others, such as changing practice structure, make it more complex to interpret. Based on information gleaned from a review of the literature, we argue that before panel size can be used as an accountability measure for individual physicians or practices in primary care, we must understand its relationship to quality and outcomes at individual and population levels, as well as the contextual factors that affect it.