Background: It is currently unknown whether early immunomodulatory treatment in relapsing-remitting MS (RRMS) can delay the transition to secondary progression (SP). less thanbrgreater than less thanbrgreater thanObjective: To compare the time interval from onset to SP in patients with RRMS between a contemporary cohort, treated with first generation disease modifying drugs (DMDs), and a historical control cohort. less thanbrgreater than less thanbrgreater thanMethods: We included a cohort of contemporary RRMS patients treated with DMDs, obtained from the Swedish National MS Registry (disease onset between 1995-2004, n = 730) and a historical population-based incidence cohort (onset 1950-64, n = 186). We retrospectively analyzed the difference in time to SP, termed the "period effect" within a 12-year survival analysis, using Kaplan-Meier and Cox regression analysis. less thanbrgreater than less thanbrgreater thanResults: We found that the "period" affected the entire severity spectrum. After adjusting for onset features, which were weaker in the contemporary material, as well as the therapy initiation time, the DMD-treated patients still exhibited a longer time to SP than the controls (hazard ratios: men, 0.32; women, 0.53). less thanbrgreater than less thanbrgreater thanConclusion: Our results showed there was a longer time to SP in the contemporary subjects given DMD. Our analyses suggested that this effect was not solely driven by the inclusion of benign cases, and it was at least partly due to the long-term immunomodulating therapy given. ; Funding Agencies|Swedish||Gothenburg Multiple Sclerosis Societies||Bayer Schering Pharma||Biogen Idec||Novartis||Sanofi-Aventis||BiogenIdec||Merck-Serono||Bayer-Schering||Teva||Swedish Research Council||Gothenburg Societies of the Neurologically Disabled||