Objectives : The amount of medical utilization by Medical Aid recipients was 3.7 times that of patients with Korean Medical Insurance. This study aims to describe the surplus medicine and the medication-related utilization, and to determine factors contributing to surplus medicine. Methods : Among those who used copayment-free Class I Medical Aid in 2005, 146,880 subjects who were $geq19$ year-old and received >365 days medical treatment per year were studied with their case managers by conducting face-to-face interviews. The analytic methods were description, chi-square, t-tests, ANCOVA and multiple logistic regressions. Results : Most subjects were female (68.6%), the elderly (62.5%), and the separated (61.6%), had an elementary graduation or less (74.8%), and had disabilities (33.2%). The percentage of subjects with surplus medicine was 18.5%. However, the percentage of females, the elderly, those with non-disabilities, the separated, the uneducated, those with a very poor perceived health status and those with an economical burden for medical treatment was 19.3%, 18.9%, 19.0%, 19.3%, 19.0%, 20.2% and 24.3%, respectively. For subjects with surplus medicine, averages for the number of used pharmacies, the pharmacy-visit days and the medication costs were 4.6 drugstores, 34.9 days and approximately 1,124 thousand Won(￦). These values were higher than those without surplus medicine (4.4 drugstores, 33.8 days, and ￦1,110 thousand, respectively). The odds ratios of the contributing factors to surplus medicine were female 1.11 (95% CI=1.07-1.14), the elderly 1.06 (95% CI=1.02-1.10), those with non-disabilities 1.08 (95% CI=1.05-1.12), the separated 1.14 (95% CI=1.10-1.18), the unmarried 1.12 (95% CI=1.07-1.18), the uneducated 1.03 (95% CI=1.01-1.08), those with a very poor perceived health status 1.04 (95% CI=1.01-1.08) and experiencing an economical burden for medical treatment 2.33 (95% CI=2.26-2.40). Conclusions : 18.5% of subjects had surplus medicine with a higher mean of medication cost. Therefore, health education and health promotion programs to prevent surplus medicine and to improve the appropriate usage of medication are necessary.