At least one third of patients enrolled in a methadone maintenance treatment program are willing to comply voluntarily with screening for and counseling about human immunodeficiency virus (HIV). A questionnaire about knowledge, attitudes, and behavior concerning acquired immunodeficiency syndrome was answered anonymously by 79% (46) of the clinical staff and 67% (868) of the enrolled patients. On their own initiative, 21% of the patients had already received voluntary anonymous HIV screening and brief counseling, seldom discussing the result with the staff. Approximately 90% of the staff and a majority of the patients (72%) thought a voluntary HIV screening program should be offered to all patients. Almost all staff (98%), but only 50% of the patients, felt the HIV test results should be known to physicians, nurses, and counselors at the clinic. Few staff members (15%) believed that patients had changed their sex behavior; more (48%) felt that needle sharing was reduced. Patients believed methadone patients in general had changed their sex behavior (49.2%) and reduced needle sharing (62%) to prevent becoming infected. Patients reported statistically significant reductions both in number of sex partners and in personal needle sharing during the past year.