The variation in resource utilization for hospitalized patients who had a group of similar disease -- a Korean Diagnosis Related Group (KDRG) -- among the same type of hospitals was studied to assess the utilization variation due to the practice pattern of hospitals. Information about inpatients who were beneficiaries of the medical insurance for teachers and government officials discharged from 20 large university teaching hospitals in Seoul during 1986 and information about the hospitals were analyzed to achieve the study objective. A total of 20,223 non-outlier patients in 100 most frequent KDRGs were included in the analysis. Case charges after the review and length of stay (LOS) were used as measures of resource utilization during a hospitalization. A substantial variation among hospitals was found in most KDRGs: the ratio of the maximum and the minimum among the mean case charges of hospitals was greater than 2 in 83 KDRGs; the difference between the maximum and the minimum among the mean case charges of hospitals was greater than 100,000 Won in 94 KDRGs; the ratio of the maximum and the minimum among the mean LOS of hospitals was greater than 2 in 82 KDRGs; the difference between the maximum and the minimum among the mean LOS of hospitals was greater than 3 days in 94 KDRGs. The practice pattern of hospitals explained more than 20% of charge variation in 49 KDRGs and more than 20% of LOS variation in 43 KDRGs. The study results indicated need for a new health policy initiative for cost containment and quality assurance.