BACKGROUND: The available data on routine immunization in Nigeria show a disparity in coverage between Northern and Southern Nigeria, with the former performing worse. The effect of socio-cultural differences on health-seeking behaviour has been identified in the literature as the main cause of the disparity. Our study analyses the role of supply-side determinants, particularly access to services, in causing these disparities. METHODS: Using routine government data, we compared supply-side determinants of access in two Northern states with two Southern states. The states were identified using criteria-based purposive selection such that the comparisons were made between a low-coverage state in the South and a low-coverage state in the North as well as between a high-coverage state in the South and a high-coverage state in the North. RESULTS: Human resources and commodities at routine immunization service delivery points were generally insufficient for service delivery in both geographical regions. While disparities were evident between individual states irrespective of regional location, compared to the South, residents in Northern Nigeria were more likely to have vaccination service delivery points located within a 5 km radius of their settlements. CONCLUSION: Our findings suggest that regional supply-side disparities are not apparent, reinforcing the earlier reported socio-cultural explanations for disparities in routine immunization service uptake between Northern and Southern Nigeria. Nonetheless, improving routine immunisation coverage services require that there are available human resources and that health facilities are equitably distributed.