The hypothalamic-pituitary-adrenal (HPA) axis activity is usually altered by heroin use. In the present study we evaluated in one hundred twenty-one heroin addicts the effects of marijuana smoking on the normalization of HPA axis upon methadone treatment. The study showed that in heroin addicts who are chronic cannabis smokers a treatment with methadone lasting 12 months was able to normalize both plasma corticotropin (ACTH) and cortisol levels, as well as to control both heroin withdrawal symptoms and opioid craving. As expected in the same group of patients marijuana smoking and its craving were not reduced by methadone treatment. Our data confirm that methadone treatment outcomes are not modified by cannabis use and they add in the literature the evidence that chronic cannabis use is not able to affect the normalization of HPA axis upon methadone treatment in heroin addicts.