This month's guest authors are affiliated with the substance abuse treatment and treatment research unit of the Connecticut Mental Health Center and the department of psychiatry at Yale University School of Medicine, where Dr. Rosen is instructor and Dr. Kosten is associate professor. They discuss a promising new treatment that may reduce abuse of cocaine by drug addicts who use intravenous heroin and may indirectly reduce risk-taking behavior associated with transmission of the human immunodeficiency virus.