to evaluate the impact on access to, and use of, health services in colombia's new national health insurance system, the authors compared two cross sections of the population: before (1993) and after (1997), with the approval of act 100, creating the general system for social security in health (sgsss). two equity indicators were assessed: concentration curves (cc) and concentration indices (ci), summarizing the distribution of access to health care and utilization of health care services provided by the sgsss according to income deciles. between 1993 and 1997, the ci for access to insurance halved from 0.34 to 0.17; simultaneously, coverage increased from 23% to 57%, especially among the poorest segments of the population, where it increased from 3.7% to 43.7% as a result of subsidies provided by local governments. the ci for utilization of health care services did not vary significantly. increased disease prevalence and utilization of services among the insured, due to biased selection of risks and moral hazards, were also documented. these findings suggest a positive impact by the reform on inequalities in access to health care insurance; however, a similar effect on inequities in utilization of health services is not clear.