This was an open clinical trial of methadyl acetate (LAAM) compared with methadone in the maintenance of 636 heroin addicts who had previously been stabilized on a maintenance regimen of methadone. The starting sample assembled by the 13 cooperating clinics were randomly assigned to continued maintenance on methadone (= 308) or crossed over to methadyl acetate (= 328) for a period of 40 weeks. The starting dose was identical to the previously established dose of methadone, but beginning with the second visit, dosage was flexible. Safety was evaluated by clinical and laboratory observations conducted at four-week intervals throughout the study. Relative efficacy was evaluated by illicit drug use, program retention and attendance, and global staff judgments. It is concluded that methadyl acetate is as safe as methadone and, when given three times a week, is an acceptable and effective maintenance drug for many heroin addicts.