Environmental influences are thought to have an aetiological role in onset of diabetes in children. Month of onset in over 2000 children in Scotland was established and there was an excess in colder/darker months than in warmer/lighter months. A meta-analysis of 21 previous studies with over 13,000 patients gave the same result at a much higher level of significance. A mechanism is postulated based on previous viral induced islet cell damage with ongoing progressive auto-immune destruction. There may be physiological seasonal changes with winter stress on carbohydrate and lipid metabolism. The raised winter levels of pituitary, adrenal and thyroid hormones fail to be antagonised by falling level of insulin. A role for seasonal variation in exercise and nutrition is considered.