To improve patient care, cost effectiveness, and resource utilization, the Illinois EMS Program attempted to regionalize emergency medical care services by identifying referral centers for critically ill and injured patients. The performance of one referral region was evaluated applying a tracer method designed to tract patients whose clinical conditions, as determined by a clinical panel, required treatment at a designated regional center. The proportion of patients reaching the appropriate centers suggests that the regional referral system under study appropriately moved only about one half the patients with the tracer conditions to the designated referral center. Additional patients were transferred to hospitals with greater emergency department capabilities, although these hospitals were no formally designated centers. Except for low birth weight, factors that might be associated with referral patterns indicate no significant differences between those patients at the regional center and those treated elsewhere.