This study aimed to measure the rates of ongoing heroin abuse among patients on methadone maintenance treatment (MMT) and sought to identify patient and treatment characteristics associated with poorer outcome. The study was carried out at an outpatient drug treatment clinic and included all patients who were on the MMT during a three month period in 2004. Treatment response was measured from analysis of opiate positive urine samples. Of the 440 patients, 63% were male and their mean age was 32 years (range 17 to 52 years). 163 patients (37%) had a comorbid psychiatric illness. The average methadone dose was 74 mg. On average, 71% of urine samples were opiate negative. Shorter time in treatment (less than 24 months), lower dose of methadone, cocaine abuse and intermittent benzodiazepine abuse were each found to be significantly associated with lower rates of opiate abstinence. Outcomes were not associated with gender, age and accessing counselling. Dual diagnosed patients tended to have higher rates of abstinence (p = 0.08). MMT clients who abuse cocaine and benzodiazepines are at increased risk of continuing opiate abuse. Higher doses of methadone might be necessary to prevent illicit opiate abuse.