Objective: To evaluate the effects of systematic co-operation among municipal employees on the number of sick-leave days per month and the type of benefit granted by the Social Insurance Office. A further aim was to evaluate the economic consequences for society.Design: A 6-year follow-up study with a matched-pairs design.Methods: Days on sick-leave were calculated for each subject one year before the intervention started and yearly for the following 6-year period. Statistical mixed-model analysis was used. The economic benefit of the intervention was estimated as the increased production stemming from fewer days on sick-leave.Subjects: Sixty-four employees on long-term sick-leave were individually matched with controls from another Social Insurance Office in a county with a socioeconomic structure similar to that of the study group.Results: The study group had 5.7 fewer days on sick-leave per month and person over the 6-year period (p=0.003). The estimated average economic benefit of the intervention was (sic)36,600 per person over the 6-year period. In conclusion, those who received systematic co-operation in vocational rehabilitation had fewer days on sick-leave than their "treatment-as-usual" peers. This effect persisted over 6 years, generating substantial net economic gains for society.