Individuals with psychoactive substance dependence suffer from high rates of non-substance use psychiatric disorders. These disorders sometimes prove difficult to distinguish in clinical practice. The Millon Clinical Multiaxial Inventory (MCMI) is a relatively new assessment tool which purports to evaluate DSM-III Axis I and Axis II disorders. In the present study, the MCMI was administered to 73 opioid addicts and 37 cocaine addicts entering treatment. MCMI protocols were then sorted four times to identify: (1) subjects with psychotic disturbances, (2) subjects with affective disturbances, (3) subjects with severe personality disorders, and (4) basic personality disorder subtypes. The covariabilities of all four categories with drug of choice are presented and discussed. The covariabilites of the first three categories with the fourth are also presented and discussed. The MCMI appears to be a useful instrument for psychiatric diagnosis of opioid and cocaine addicts.