When affordable insurance becomes available do previously-uninsured populations use more health care services since only those who need care will opt to obtain coverage? This pilot study assessed the feasibility of evaluating this question by analyzing the utilization/disenrollment patterns of low-income, previously-uninsured individuals in three RWJ demonstration sites compared with those of commercially-enrolled individuals in the same health plans. The pilot study determined that critical data could be collected by telephone survey from enrollees and disenrollees and that data from two of the three HMOs are complete and compatible enough for rigorous analysis.