The incidence of type 1 diabetes (T1D) has during the last few decades been increasing in children and juveniles. Multi-factorial courses combining genetic disposition and environmental factors might be in play, and through the years, there has been a mounting interest in the innate immune system's role in the development of T1D. The aim of this study was to determine mannose binding lectin (MBL) levels in newly diagnosed children with T1D (n=481) over a period of 10 years (1997-2005) and to compare these levels with corresponding levels in their healthy siblings (n=479). Furthermore, the aims were to evaluate if MBL-levels in patients and siblings were influenced by season, age autoimmunity and/or changed over time. The study found that MBL levels differed between patients and their healthy siblings when adjusted for age, gender, season and period. More patients than siblings had MBL levels above 0.8 μg/ml, associated with high producing MBL genotypes, and the elevated MBL levels were associated with high levels of four T1D related cytokines (IL-1β, IL-12, IL-18 and TNF-α). MBL levels increased during the study period and siblings had seasonal variance in concentrations with the lowest level during wintertime (Dec-Feb). In conclusion, more patients than siblings had a high MBL level, and high levels of MBL were related to high levels of T1D specific cytokines, supporting a role of the innate immune system and MBL on the risk of developing T1D.