RESUMEN
Understanding the effect of different factors on the utilization of physician services is of great importance for policy making processes in health sector. Identification of such factors will improve the process of planning and policy making in health sector. To examine the effect of accessibility, income and out- of- pocket payments for visit and medicine on the utilization of specialist doctors' services [visits], in the population covered by Social Security Organization [SSO]. Managerial data bases throughout the SSO were used to measure the effect of assumed variables on the utilization of specialist doctors' services [visits] in a thirty-million population covered by SSO. The data were collected from 1998 to 2009 and then adjusted for the purpose of the study. A bilogarithmic regression model with panel data, using fixed effect model was estimated. The study results showed that 1% increase in the geographic accessibility and medicine costs would increase the utilization of specialist doctors' services by 23% and 17%, respectively. As well, out-of-pocket payments for specialists' visits and the insureds' incomes have no significant effect on the utilization of these services. Visiting specialists in private sector showed no responsiveness to out-of- pocket payments, but more for geographic accessibility. Social Security Organization can regulate and control the utilization of these services through considering the flexibility of different provinces to different factors.