Methadone HCl is currently legally dispensed to approximately 80,000 heroin addicts in the United States. It is widely heralded as being virtually free from other than narcotic side effects. This report describes in detail three patients with massive fluid gain in a population of 420 addicts treated at the Hope Center methadone program in Tucson, Arizona. In no patient was there a prior history of edema, and an intensive search failed to reveal a cause for the swelling. Weight gain uniformly began 3 to 6 months after initiation of methadone or a sharp increase in the dosage, and in all three patients during periods when they sharply reduced or discontinued methadone, edema was resolved associated with a concomitant loss of weight.