Methadone maintenance treatment programs (MMTP) often provide onsite primary care. It is unclear whether patients in these settings consider the MMTP their usual source of care. We conducted cross-sectional interviews of 62 adults in an inner-city MMTP with onsite primary care to determine their usual source of care. Program enrollment ranged from 1 to 27 years (median, 4) and 63% attended > or = 5 days per week. Seventy-six percent had > or = 1 chronic disease. Only 53% reported having a usual source of care, which included hospital-based clinics (45%), the MMTP (23%), private physicians (19%), other sites (13%). Patients were more likely to identify the MMTP as their usual source of care if they had cardiovascular disease (RR 6.9, 95% CI 2.2 to 21.9) or HIV (RR 5.6, 95% CI 1.7 to 18.5). Successfully promoting appropriate utilization of onsite primary care may require a better understanding of MMTP patients' perceptions of primary care.