Background: Impact of changing diagnostic criteria for the population-based incidence of multiple sclerosis (MS) has not been investigated.

Objective: To assess the effect of changing diagnostic criteria on national MS incidence and prevalence in Finland from 1974 to 2021.

Methods: We identified patients with MS (pwMS) through the National MS registry and the national Care Register for Healthcare and divided them into four groups based on the year of MS diagnosis: 1) Schumacher criteria (1974-1982), 2) Poser criteria (1983-2000), 3) Earlier McDonald criteria (2001-2016), and 4) Current McDonald criteria (2017-2021). Age-adjusted incidence and prevalence were calculated.

Results: Age-adjusted incidence per 10⁵ person years increased from 3.7 (95% CI 3.5-3.8) during the Schumacher criteria period to 9.2 (95% CI 9.0-9.4) during the earlier McDonald criteria. During the Current McDonald criteria incidence stabilized to 8.6 (95% CI 8.3-9.0). Prevalence increased from 24.3 (95% CI 22.8-25.8) to 241.5 (95% CI 237.3-245.6) per 10⁵ person years.

Conclusion: Both incidence and prevalence of MS increased significantly. Incidence showed a sharp increase when entering the twenty-first century, after which it stabilized. Increasing incidence was likely related to incorporation of MRI in the diagnostic criteria. Current diagnostic criteria did not further increase the incidence.