Background. There has been noticed a systematic growth of using psychoactive substance (SP) in last years. The co-occurrence of mental and physical disorders related to substance abuse of treated patients is more often a serious problem to medical services. Dual diagnosis (DD) is a clinical term referring to co-morbidity or the co-occurrence in the same individual of a psychoactive substance use disorder and another psychiatric disorder. The aim of the study is to investigate the prevalence of dual diagnosis in patients with diagnosis of substance use disorder hospitalized in years 1994-2005, to assess the kind of co-morbid mental disorders and the course of treatment in three groups: patients with DD, with diagnosis of mental disorder without substance use and with diagnosis related to substance use. Methods. The retrospective study of 4 349 case records of patients hospitalized in the department of psychiatry in years 1994-2005. Out of this number two groups of patients were separated: persons abusing or dependent on SP (n = 825) and patients with dual diagnosis (n = 362). The control group (n = 200) was created among patients with mental disorders and without SP abuse. Socio-demographic factors, number and the length of hospitalizations, aggressive behaviours, suicide attempts, discharges from hospital on demand were analyzed. In the DD group there was an attempt to evaluate the relation between substance use disorders and co-occuring mental disorders performed. Results. The frequency of DD among all patients hospitalized in the studied period of time was 8.3%, whereas among patients abusing SP was 30.5%. This study demonstrates that patients with the DD are statistically longer hospitalized, discharged from hospitals at their own request and more often need treatment in hospitals, statistically more often try to commit suicide and perform aggressive behavior. Mental disorders were substantially often secondary to substance related disorders in the DD group. There was proved that patients mainly abused alcohol and the most frequent mental disorder were mood (affective) disorders.