Context: In Finland the world-record high incidence of type 1 diabetes has risen steeply over past decades; however, after 2006 the incidence rate has plateaued. We showed earlier, that despite the strong genetic disease component, environmental factors are driving the increasing disease incidence. Objective: Since vitamin D intake has increased considerably in the country since 2003, we analyzed how serum 25-hydroxyvitamin D (25[OH]D) concentration changed over time in healthy children, and the timely relation of these changes to disease incidence. Design, Setting and Participants: The birth cohort of the Finnish Type 1 Diabetes Prediction and Prevention project was used to explore longitudinal changes in serum 25-hydroxyvitamin concentrations. The sampling period was limited to children born during 1994-2004 with serum samples collected during 1998-2006 in Turku area - Southwest Finland (60 °N). Main Outcome Measure: 25(OH)D concentrations were measured every 3-6 months from birth - age range 0.3-12.2 years (387 subjects; 5334 measurements). Results: Serum 25(OH)D concentrations were markedly lower before 2003 than thereafter (69.3±1.0 nmol/L vs. 84.9±1.3 nmol/L, respectively, P < .001) in both genders. The mean difference between the periods was 15.7±1.3 nmol/L (P< .001). Importantly, the frequency of children with low serum 25(OH)D levels (< 50 nmol/L) was reduced to almost half from 2003 (37.3 % versus 69.9 %; P< .001). Similarly, severe vitamin D deficiency (<25 nmol/L) has also decreased (2.7% vs. 7.7%; P = .005). In addition, we detected higher 25(OH)D concentrations in young children (<2 years) as compared to older children, which is explained by higher vitamin D intake in this group. Conclusions: We provide evidence that an increase in circulating concentrations of 25(OH)D shows a delayed temporal association with leveling off of type 1 diabetes incidence in Finland after 2006.