Therapeutic drug monitoring of Mycophenolate mofetil(MMF) has been suggested in some clinical trials, but has not been widely adopted in Korea. The purpose of this study was to analyze the withdrawal rates of MMF and determine the characteristics of the patients who experienced adverse reactions with MMF therapy and to suggest the criteria for selecting patients who need monitoring of MMF levels. We retrospectively collected data of patients who started MMF between July 2007 and June 2008. A total of 154 adult patients were included in our study. Among them, ninety seven patients discontinued MMF with 59 cases being due to adverse drug reactions. Thirty one patients required dosage reduction of MMF with twenty three cases being due to adverse reactions. Twenty six patients continued the MMF without or with mild adverse reactions. Of the 82 adverse reaction cases, hematologic adverse reactions accounted for 38 cases (46%) and gastrointestinal (GI) adverse reactions accounted for 28 cases (34%). Older age and lower serum albumin levels were significantly different characteristics between the patients who withdraw MMF due to hematological adverse reactions and those who were able to continue therapy. The group who experienced GI adverse reactions had higher MMF dosages based on body weight and lower serum albumin levels. In conclusion, the factors affecting the adverse reactions of MMF were age, serum albumin level and higher dosage, therefore therapeutic drug monitoring of MMF should be considered in these patients.