Background & objectives: Creation of a strong referral transport network across the country is necessary for improving physical access to public sector health facilities. In this study we evaluated the referral transport services in Haryana, i.e. Haryana Swasthya Vaahan Sewa (HSVS), now known as National Ambulance Service (NAS), to assess the extent and pattern of utilization, and to ascertain its effect on public sector institutional deliveries. Methods: Secondary data on 116,562 patients transported during April to July 2011 in Haryana state were analysed to assess extent and pattern of NAS utilization. Exit interviews were conducted with 270 consecutively selected users and non- users of referral services respectively in Ambala (High NAS utilization), Hisar (medium utilization) and Narnaul (low utilization) districts. Month-wise data on institutional deliveries in public facilities during 2005-2012 were collected in these three districts, and analysed using interrupted time series analysis to assess the impact of NAS on institutional deliveries. Results: Female gender (OR = 77.7), rural place of residence (OR = 5.96) and poor socio-economic status (poorest wealth quintile OR = 2.64) were significantly associated with NAS ambulance service usage. Institutional deliveries in Haryana rose significantly after the introduction of NAS service in Ambala (OR=137.4, 95% CI=22.4-252.4) and Hisar (OR=215, 95% CI=88.5-341.3) districts. No significant increase was observed in Narnaul (OR=4.5, 95% CI= -137.4 to 146.4) district. Interpretation & conclusions: The findings of the present study showed a positive effect of referral transport service on increasing institutional deliveries. However, this needs to be backed up with adequate supply of basic and emergency obstetric care at hospitals and health centres.