Patients in the most prevalent DRGs in a Medical Intensive Care Unit (MICU) were compared with their counterparts who received only routine hospital care on adjusted total hospital costs and length of stay. Costs for both groups were compared with estimated DRG payments under an all-payer system. For patients in three DRGs, measures of severity of illness were examined as predictors of costs. Significant differences between MICU and routine care patients were found in 10 of 13 DRGs studied; intensive care costs were substantially above overall payment rates. The severity of illness measures varied widely in their correlation with costs, depending on DRG and whether the patients were MICU or routine care. These apparent differences in accounting costs may result in hospital decisions to restrict the number of MICU beds. Severity of illness adjustments to DRGs might produce more equitable payments. The most useful measure of severity may differ, however, depending on DRG.