Drug-drug interactions increase the effectiveness of therapy but can also have undesirable effects on toxicity. A pharmacist has the responsibility to prevent the undesirable effect of drug-drug interactions. The purpose of this study was to evaluate the potential drug-drug interactions in hypercholesterolemia patients at Teaching Hospital Surabaya. This study was conducted with a prospective method for a month. We obtained 54 patients and identified 45 patients that have potential drug-drug interactions. The total potential drug-drug interactions were 101. The severity of potential drug-drug interactions is serious (24,7%) and moderate (75,3%). Based on the mechanism action of drug interactions are pharmacodynamics (59,4%), pharmacokinetics (32,7%), and unknown (7,9%). The role of the pharmacist in providing safe, effective and rational drug therapy can be done by either identifying drug interactions, prevent and minimize the negative effect of drug interactions.