Methadone has been used as an alternative strong opioid to morphine in the management of cancer pain. The conversion of morphine to methadone is not straightforward because of the high individual variability and unpredictability in the pharmacokinetics of methadone. An ad libitum schedule for conversion of morphine to methadone was used in 37 cancer patients who had intolerable morphine-related side effects or had pain not satisfactorily controlled by morphine. Oral morphine was discontinued on the day of conversion. Methadone was given at a dose calculated as one-twelfth of the total daily dose of morphine, up to a maximum of 30 mg/dose. Methadone was administered at patient-controlled intervals not more frequent than three hours, the need of which was indicated by the presence of pain of moderate intensity or above as rated by a verbal rating scale. When the demand for methadone was stabilized, the total daily dose was given regularly in divided doses. Pain control on day 7 was taken as the primary endpoint. Twenty-seven patients completed the study. Twenty-four patients (88.9%) were in good pain control on day 7, and all reached good pain control by day 11. The median time required to achieve good pain control was three days (range 1-11 days). A majority (88.6%) of morphine-related adverse effects improved or resolved after conversion to methadone. This ad libitum schedule is effective in conversion of morphine to methadone in these patients.