AIMS: The study estimates the incidence of psychiatric disorders in long-term sickness absence (LSA; more than eight weeks of continuous sickness absence) over one year. The study is the first accounting for everybody on LSA by linking a psychiatric assessment for all persons on LSA to public registers. METHODS: In a Danish population of 120,000 inhabitants all 2,414 incident persons on LSA within one year were posted a questionnaire, of whom 1,121 (46.4%) responded. In a two phase design the 1,121 sick-listed persons were screened for psychiatric disorders. Phase 2 consisted of 844 people with predefined scores on CMD-SQ of whom 423 were randomly invited to a psychiatric examination and 329 participated. A further 11 who did not fulfil the predefined criteria were invited, of whom eight participated. The examination included a diagnostic interview by Present State Examination. RESULTS: The 329 examined persons in Phase 2 showed by binomial tests the following frequencies: any psychiatric disorder 57%, any depression 42%, and any anxiety 18%. In Phase 1, representative for everyone on LSA, the frequencies were 48% for any psychiatric disorder, 35% for any depression, 15% for any anxiety, and 7% for any somatoform disorder. Multivariable analyses showed that female sex and unemployment were predictors of a psychiatric disorder, whereas living with children below 18 years and being a skilled worker carried a reduced risk of a psychiatric disorder. CONCLUSIONS: The high frequency of psychiatric disorders necessitates further population-based studies documenting the efficiency of screening and psychiatric examinations in LSA.