Our goal consisted of describing the 4-year prevalence, contributors, and interventions for long QTc's in methadone maintenance treatment. Cardiologists' diagnosis of long QTc defined case-ness in 62 patients. Long QTc categories, drawn from epidemiological reports, encompassed 440 to 469 (borderline), 470 to 499 (moderate), and 500+ milliseconds (severe). Data collection included electrocardiograms, demographic characteristics, contributors to long QTc, and interventions-plus-outcomes (defined by resolution of long QTc). Of 62 patients, 21 had 39 long QTc episodes—a 4-year case prevalence of 34%, and an annual episode incidence of 15.7 per 100. Contributing factors identified in 36 of 39 episodes consisted of medication management (n = 19), illicit drug use (n = 11), and other factors (n = 6). Long QTc reverted to normal in 38 of 39 episodes. Of 21 patients, 12 (57%) experienced one or two recurrences. Methadone maintenance treatment physicians normalized most episodes as outpatients, often in collaboration with patients' primary care physicians. One fifth of episodes required hospitalization and other specialty care. Lack of timely QTc normalization may have accounted for one sudden death.