The aim of this epidemiological study was to investigate the prevalence of psychological and somatoform dissociation and associated factors in the general population. The course of psychological dissociation was examined in a three-year follow-up study. Dissociation was measured with the Dissociative Experiences Scale (DES) and its subscale for pathological dissociation, the Dissociative Experiences Scale Taxon (DES-T), and with the Somatoform Dissociation Questionnaire (SDQ-20). Participants in the study (n = 3004) were derived from a random sample of general population adults (25-64 years old) in eastern Finland. The data were gathered by postal questionnaires in 1998, 1999 and 2001. The response rate was 68% at baseline, and 75% of the baseline respondents returned the questionnaire on three-year follow-up in 2001.