The authors conducted an empirical test of the relationship between physician supply and hospitalization for ambulatory care sensitive conditions (ACSH). Principal findings showed that in the urban models for ages 0-17 physician supply has the largest negative adjusted relationship with ACSH. For ages 18-39 and 40-64, physician supply has the second largest negative adjusted relationship with ACSH. Physician supply was not associated with ACSH in rural areas. The authors conclude that physician supply is positively associated with the overall performance of the primary health care system in a large sample of urban counties of the United States.