There are limited number of studies about dissociative disorders and substance abuse/dependence. Ross et al (1992) found that 39.0% of 100 chemically dependent patients had a dissociative disorder. Dunn et al (1995) found that this rate was 15.0% for 100 patients with substance use disorder at the end of their inpatient treatment. In the third study, 215 consecutive inpatients were recruited. The prevalence of the DSM-IV dissociative disorders was 17.2%. 67.6% of these patients’ dissociative experiences had started 3.6 (SD=2.9, range: 1.0-11.0) years before onset of the substance use in average. Patients with dissociative disorder were younger and average duration of their remission periods was shorter. Dissociative patients tended to use more than one substance whereas drugs were used more frequently than alcohol in this group. The frequency of borderline personality disorder, somatization disorder, history of suicide attempt, and childhood abuse and neglect in this qroup was more than that in the non-dissociative group. Suicide attempt, female gender, and childhood emotional abuse were significant predictors of a dissociative disorder diagnosis. Significantly more patients with dissociative disorder stopped their treatment prematurely. These findings suggest that dissociative disorder comorbidity is not a phenomenon limited to a cross-sectional observation and to a crisis period, but it is involved in all phases of substance dependency and it may have tremendous impact on the course and treatment outcome of the process.