The Hospital Admission and Surveillance Program (HASP) was a pre-PSRO program in Illinois. Established in February 1972, its objective was to reduce the cost of care given to Medicaid beneficiaries in that state. HASP approved length of stay (L.O.S.) on a prospective only basis, so that all Medicaid patients were reported promptly on admission. During 1972, the Illinois Medicaid program was rapidly expanding in patient volume, but not in scope of services. This study compares patients admitted to 46 hospitals in Illinois before and after the establishment of HASP. To adjust for changes in case mix between the two periods, each patient was matched for age, diagnosis and illness severity with patients in an appropriate comparison group. The complex process of matching raised the likelihood that changes in L.O.S. were caused by HASP's activities and not by other factors. L.O.S. fell significantly more for Medicaid than for non-Medicaid patients. In addition, the mix of diagnoses changed more markedly for Medicaid than non-Medicaid patients, emphasizing the need for case mix adjustments in this before-and-after study. Some findings suggest that the “post-HASP” admissions had the greater need for hospital care; this trend affected both Medicaid and non-Medicaid patients, however. The combined evidence suggests that concurrent review, as implemented by HASP, helped shorten L.O.S. in 1972. Whether quality of care changed was not studied.