Different instruments are used in clinical practice to assess comorbid psychopathology in addicted individuals. This study is aimed at comparing two of those instruments. In total, 327 heroin- and methadone-addicted individuals were interviewed in three treatment settings and outside treatment. Instruments used are the Addiction Severity Index (ASI) and the Composite International Diagnostic Interview (CIDI). The former instrument results in a general measure of severity of psychopathology, while the latter results in categorical DSM-III-R diagnoses. A comparison of the results show, however, that the two types of data do not agree to a large extent. By using the DSM-III-R data as golden standard, it appeared that a part of the psychopathology cases was missed out by the ASI severity measures. The results, that are especially of interest for clinicians using the ASI, are presented for various disorders.