Using the health care system fully in some countries requires patients to register with a primary care physician (PCP). Public health policies measure PCP density to maintain satisfactory local PCP supplies and limit geographic inequalities. In an exhaustive simulated-patient survey in the Paris, France region, we analyzed how well presence of PCPs was associated with patients’ ability to register for care. Of 5,188 census blocks, 55.4% had at least 1 PCP; however, only 38.6% had at least 1 PCP accepting registration for office visits, and only 19.4% had at least 1 PCP accepting registration for home visits (P <.001 across the 3 indicators). Cross-block inequalities in accepting registration were steeper than those related to PCP density, indicating that this density metric offers false reassurance and is inadequate to support policy decisions.