After informed consent and screening, HIV infected individuals who are cocaine dependent and qualify for the study will be offered ondansetron 4mg BID for six weeks in an open label format 4mg BID has been found to have efficacy compared to placebo. At screening and then at each visit, they will be asked to provide urine and a drug of abuse screen will be conducted to assess for cocaine. They will be asked to detail their recent cocaine use in the last month and then will be given a visual analog scale to assess their craving for cocaine. They will be asked to return weekly for 6 weeks to receive a week's supply of ondansetron and to give a urine sample that will test for cocaine. They will fill out a time line follow back for the past week and asked to assess their craving for cocaine on a visual analog scale.