Summary The merging of symptoms of substance-use disorders with symptoms of other psychiatric disorders and the occurrence of dual diagnosis is investigated on the basis of data from 5 Belgian general psychiatric hospitals. An analysis of data for 6263 admissions for treatment revealed high percentages of comorbidity, particulary of comorbidity with personality disorders. The validy of the results is however questionable because of inherent weaknesses in the official registration systems (the Minimum Psychiatric Data, and the Flemish Registration of Substance Use). It is recommended that the current registration systems be adapted and improved.