There is a widespread perception that the increasing proportion of female physicians in most developed countries is contributing to a primary care service shortage because females work less and provide less patient care compared with their male counterparts. There has, however, been no comprehensive investigation of the effects of primary care physician (PCP) workforce feminization on service supply. We undertook a systematic review to examine the current evidence that quantifies the effect of feminization on time spent working, intensity and scope of work, and practice characteristics. We searched Medline, Embase, and Web of Science from 1991 to 2013 using variations of the terms 'primary care', 'women', 'manpower', and 'supply and distribution'; screened the abstracts of all articles; and entered those meeting our inclusion criteria into a data abstraction tool. Original research comparing male to female PCPs on measures of years of practice, time spent working, intensity of work, scope of work, or practice characteristics was included. We screened 1,271 unique abstracts and selected 74 studies for full-text review. Of these, 34 met the inclusion criteria. Years of practice, hours of work, intensity of work, scope of work, and practice characteristics featured in 12%, 53%, 42%, 50%, and 21% of studies respectively. Female PCPs self-report fewer hours of work than male PCPs, have fewer patient encounters, and deliver fewer services, but spend longer with their patients during a contact and deal with more separate presenting problems in one visit. They write fewer prescriptions but refer to diagnostic services and specialist physicians more often. The studies included in this review suggest that the feminization of the workforce is likely to have a small negative impact on the availability of primary health care services, and that the drivers of observed differences between male and female PCPs are complex and nuanced. The true scale of the impact of these findings on future effective physician supply is difficult to determine with currently available evidence, given that few studies looked at trends over time, and results from those that did are inconsistent. Additional research examining gender differences in practice patterns and scope of work is warranted.