Two controlled trials have suggested that clonidine can be used on an outpatient basis to assist in opioid withdrawal, but that rates of successful detoxification do not exceed those obtained with gradual methadone reduction. This study examines the process of clonidine-and methadone-assisted detoxification to provide information about the timing and severity of withdrawal symptoms and side effects associated with each of the two methods. Derived from a double-blind clinical trial with randomized treatment assignment, data are presented which might guide the clinician in the choice of detoxification methods. In this study, subjects in either treatment condition had a success rate of approximately 40% and most subjects experienced at least moderate withdrawal discomfort in the areas of difficulty sleeping, feeling "blah," and craving. In both treatment groups the best predictor of detoxification failure was a high level of psychological symptoms at the onset of the study. Compared with gradual methadone reduction, clonidine treatment resulted in higher levels of withdrawal symptoms and side effects, earlier onset of withdrawal discomfort, earlier attrition, earlier termination of withdrawal discomfort, and a posttreatment course of drug use that was more consistent with success status during the study treatment.