Mental illness impacts work performance and sickness absence. An important objective is to find out whether mental illness also influences the duration of sickness absence. the aim was to assess number of sick-leave days in sick-listed individuals with mental illness compared to those without. the study base was the general population aged 18–64 years in Västra Götaland, Sweden. all individuals who became sick-listed during the period February 18 to April 15 in 2008 were invited and 3310 participated. Questionnaire data on two indicators of mental illness constituted baseline data. Duration of sickness absence for all psychiatric and somatic diagnoses was derived from the national sickness absence register. Duration of sickness absence data was categorized as follows: ≤14 sick-leave days (n = 293), 15–28 days (n = 776), 29–104 days (n = 979) and 105 days or longer (n = 615). the final study population consisted of 2663 individuals, 66% women. Preliminary descriptive data showed that a higher proportion of individuals (38%) that scored low on the WHO (ten) well-being Index was found in the category ≥105 sick-leave days than those who scored high (17%). the differences were even a bit stronger when self-reported mental disorder was used as indicator: 43% was found in the category ≥105 sick-leave days compared to 21% in the group without mental disorders. the presence of mental illness prolonged duration of sickness absence in this prospective general population study of incident sick-leave cases.