The concomitant use of cocaine by heroin-dependent subjects, or by patients on methadone maintenance treatment, is a relevant phenomenon that determines the negative consequences on health, social adjustment, and outcome of opioid addiction treatment. Little is known about the patterns of co-use of these two substances and the pathophysiological alterations following this condition. Only a few studies have evaluated the neurochemical effects in subjects carrying this specific pattern of abuse. Similarly, the impact of cocaine abuse on psychiatric and social function in subjects already affected by opioid addiction is still poorly understood and further studies are necessary to investigate this specific area that could profoundly affect methadone maintenance treatment. The aim of this article is to investigate the psychopathological symptoms of heroin-cocaine abuse in a group of heroin addicts applying for treatment. Results show a direct relationship between cocaine abuse and a higher rate of psychiatric disorders, but a negative correlation with the severity of self-rated psychopathology.