The aim of this study was to investigate mental illnesses among the substance abuse dependent populations. A total of 1076 substance abusers were recruited from the Outpatient Department of the Central Drug Addiction Treatment Center, Tejgaon, Dhaka from July 2008 to June 2009. They sought detoxification therapy voluntarily at this centre. The research participants were selected consecutively following the defined selection criteria. Research instruments were interviewer-administered questionnaire and standard mental state examination scales. Of the 1076 substance abusers, 82.6% had been using heroin currently and rest of them used phensedyl followed by injection drugs and cannabis with a period ranged 2-30 years. Results showed that 91.3% of the substance abusers had been suffering from insomnia and 75.0% had altered food habit. About 49.0% showed disturbed behaviors and 45.2% had been suffering from sexual dysfunctions. Around 32.0% of the substance abusers had been suffering from nonspecific generalized anxieties and 72.7% were found in abnormal mood/affects. A striking finding was that 7.3% of the substance abusers had been suffering from perceptual and/or thought disturbances. In conclusion, 7.3%-92.5% of the substance abusers had been suffering from mental illnesses. Insomnias, decreased intake of food and taste preference, irritable mood/affects, loss of interest in sex and non-specific anxieties were highly prevalent among them. Medical management and altering lifestyle are still the only applicable way to control this human catastrophe.