Private practice physicians in New York City’s poorest neighborhoods are typically foreign trained, have generally substandard clinical practices, and have been accused of rushing Medicaid patients through to turn a profit. However, they also represent a sizable share of physician capacity in medically underserved neighborhoods. This article documents the level of credentials, systems, and immunization-related procedures among these physicians. Furthermore, it assesses the relationship between such characteristics and childhood immunization rates. The analysis utilizes a cross-sectional comparison of immunization rates in 60 private practices that submitted 2,500 or more Medicaid claims for children. Immunization data were gathered from medical records for 2,948 randomly selected children under 3 years of age. Half of sampled physicians were board certified (55%), and half were accepted by the Medicaid P referred P hysicians and Children (P P AC) program (51.7%). Of physicians, 43% saw patients only on a walk-in basis, while only 17% scheduled the next appointment while the patient was still in the office. There were 75% of the physicians who reported usually immunizing at acute care visits. Immunization rates were higher among PPAC physicians compared to others (41% vs. 29% up to date for diphtheria and tetanus toxoids and pertussis [DTP]/Haemophilus influenzae type b [Hib], polio, and measles-mumps-rubella [MMR], P=.01), and board-certified physicians showed a trend tovard better immunization rates (39% vs. 30%, P=.07). Physicians who reported usually immunizing at acute care visits also had higher rates than those who did not (38% vs. 27%, P=.05). Scheduling a date and time for the next immunization showed a trend toward association with immunization coverage (37% vs. 28%, P=.10). Private practice physicians who provide high volumes of care reimbursed by Medicaid have improved their credentials and affiliations over time, thereby expanding reimbursement options. Credentials and affiliations were at least as effective in distinguishing relatively high- and low-performing physicians, as were immunization-related practices, suggesting that they are useful markers for higher quality care. The relative success of the P P AC programshould informefforts to inprove the capacity and quality of primary care for vulnerable children. Appointment and reminder systems that effectively manage the flow of children back into the office for immunizations and the vigilant use of acute care visits for immunizations go hand in hand. Opportunity exists for payers and plans to encourage and support these actions.