Background: Patients diagnosed with cancer tend to have multiple risk factors for drug related problems such as old age, comorbid conditions, self-medication, additional medications for supportive therapy, and the cancer treatment itself. Objective: The aim of this study was to investigate notable polypharmacy and related problems in oncology patients during hospitalization or following clinic visit. Method: The electronic medical records of patients who were discharged from oncology division at Seoul National University Hospital (SNUH) from June $1^{st}$ to June $30^{th}$, 2011 were retrospectively reviewed. Results: A total of 334 discharged patients were included in this study. Among them, 221 patients had pre-admission medications. Forty percent of the patients, including 60.3% of elderly patients (over 65 years of age) were prescribed more than 5 kinds of medications. Twenty percent of the patients were prescribed to take medications more than 7 times a day. Medication duplications were observed in 2.4% of patients. In 10.5% of the reviewed patients, 47 cases of potential drug-drug interaction including 3 cases of contraindication and 17 cases of major clinical implication according to Micromedex were detected. Medication changes were made in 88% of patients during hospitalization and in 94.7% of patients at their first visit after discharge. Antidiarrheal agent (loperamide) was prescribed along with laxatives in 18.5% of the prescriptions and inadequate instructions on how to take it were observed in 63% of the prescriptions. Conclusion: This study identified the prevalence of polypharmacy and related problems in cancer patients, which presents the need for an active role of pharmacists in pharmaceutical care in oncology wards.