Drug interaction may cause sever adverse drug reactions in patients. The high amounts of prescribed medication in hospitalized patients will cause higher incidence of drug interactions. Therefore, the study of drug interactions in hospitalized patients seems to be worth. We have, studied drug interaction in internal and surgical units, in Kashani hospital in Shahrekord. Factors included in this study were: mean of prescribed medications, frequency of drug interactions, correlation between these two parameters, and the most frequent drug interactions. The frequency of drug interactions in different units of hosital, was as follows: 26.4 percent in surgical unit, 30.7 percent in internal unit and 34 percent in cardiac unit. The mean of prescribed medications was as follows: 3.90 in surgical unit, 3.92 in internal unit and 5.86 in cardiac unit. There was a positive correlation between the number of medications and the frequency of drug interactions in prescriptions. Six different drug interactions, occurred frequently. Therefore, it seems if physicians consider these interactions, the percent of drug interactions would be reduced considerably. We also found that most of drug interactions occurred when two or more diseases were managed concurrently. Therefore, it seems, having attention to these cases, would cause drug interactions to be reduced significantly.