Blood pressure and heart rate were monitored in heroin addicts detoxified with guanfacine in an outpatient program. Retrospective analysis of the cardiovascular parameters from the addicts who failed this detoxification during the first 5 days of treatment (they consumed heroin within this period) showed that these patients experienced less hypotension and bradycardia when treated with the alpha 2-agonist than the group of successful patients. Therefore, the relapse to opiate use could be predicted, at least in some patients, by a minimal cardiovascular reaction to guanfacine. Opiate use was followed by significant increases in blood pressure and heart rate, as had happened in previous studies with clonidine-treated addicts. This finding further confirms a similarity between alpha 2-agonists adrenergic and opiates with regard to their cardiovascular effects.