Background and Objectives: Electrocardiogram (EKG) monitoring during methadone maintenance treatment (MMT) has been recommended to prevent potentially fatal prolonged computed QT intervals (QTc). However, risk indicators for obtaining EKGs do not exist. This study assessed 23 variables that might help identify prolonged QTc during MMT. Methods: EKGs concurrent with methadone serum levels were obtained from 69 veterans during a 5-year study, encompassing 302.8 person-years. Two cardiologists hand-measured QT intervals, selecting each patient's longest QTc. QTc categories included: normal duration <440 ms; borderline duration of 440–469 ms; and abnormal duration ≥470 ms. QTc's were compared with seven methadone parameters and 16 bio-psycho-social variables using two QTc cut-offs (440 and 470 ms). Results: Among the 69 patients, 19 had normal QTc's, 28 had borderline QTc's, and 22 had abnormal QTc's. Methadone dose/weight was moderately correlated with QTc, and independently associated with longer QTc at both 440 and 470 cut-offs. Discussion and Conclusion: Dose/weight ≥.49 is useful for screening EKGs for QTc's ≥440 cut-off. Dose/weight ≥.65 produces high-yield abnormal QTc's ≥470 cut-off. Scientific Significance: Methadone dose/weight provides moderately reliable thresholds for making routine screening decisions and urgent clinical decisions to obtain an EKG for prolonged QTc. (Am J Addict 2016;25:499–507).