Cancer patients are more susceptible to adverse drug-drug interactions (DDIs) due to receiving multiple medications especially chemotherapy medications, hormonal agents and supportive care drugs. The aim of this study is to describe the prevalence of potential DDIs and to identify risk factors for these potential interactions in hospitalized cancer patients in a developing country.A cross-sectional study conducted by reviewing charts of 224 consecutive in hospitalized patients in hematology-oncology ward of a teaching hospital in Tehran, during a 12 month period from July 2009 to July 2010. “Drug Interaction Facts 2008, 2009: The Authority on Drug Interactions” was used for screening the potential drug-drug interactions. Potential interactionswere classified by levels of severity and documentation.The median age of patients was 50 years, the length of hospital stay for patient was 5 days and the number of drugs per patient was 8 drugs. Two hundred and twenty-eight potential interactions were detected. Nearly 14% of the interactions were major and 60% were moderate. Approximately 9% and 10% potential interactions were graded as established and probable. In multivariate analysis, being older than 61 years old, suffering from hematologic cancer, source of cancer in different specific organs (esophagus, testis and cervices more than other sources), and number of ordered drugs for patients were independent predictors of having at least one potential DDI in hospital order. Suffering from hematologic cancer, source of cancer in different organs, length of hospital stay and number of ordered drugs for patients were independent predictors for number of interactions per patients.Having a DDI seems to be more likely to occur in patients older than 61 years old. Hematologic cancers, having more medications in physician’s order, longer length of hospital stay, esophageal cancer, testicular cancer and cervical cancer have related to having a DDI and also having more number of interactions.