A consecutive sample of patients who were switched from strong opioids to methadone in a period of 1 year was surveyed. QTc was assessed before switching (T0) and after achieving adequate analgesia and an acceptable level of adverse effects (Ts). Twenty-eight of 33 patients were switched to methadone successfully. The mean initial methadone doses at T0 were 67.1 mg/day (SD +/-80.2, range 12-390). The mean QTc interval at T0 was 400 ms (SD +/-30, range 330-450). The mean QTc interval at Ts (median 5 days) was 430 ms (SD +/-26, range 390-500). The difference (7.7 %) was significant (p < 0.0005). Only two patients had a QTc of 500 ms. No serious arrhythmia was observed. At the linear regression analysis, there was no significant association between mean opioid doses expressed as oral morphine equivalents and QTc at T0 (p = 0.428), nor between mean methadone doses and QTc at Ts (p = 0.315). No age differences were found with previous opioid doses (p = 0.917), methadone doses (p = 0.613), QTc at T0 (p = 0.173), QTc at Ts (p = 0.297), and final opioid-methadone conversion ratio (p = 0.064). While methadone used for opioids switching seems to be an optimal choice to improve the opioid response in patients poorly responsive to the previous opioid, the possible QTc prolongation should be of concern despite not producing clinical consequences in this group of patients. A larger number of patients should be assessed to quantify the risk of serious arrhythmia.