ObjEctIvE: the aim of the study was to investigate the seasonal variation of type 1 diabetes mellitus (t1DM) diagnosis in Greek children. DEsIGN: the study group consisted of 1148 patients (604 males and 544 females) who were diagnosed with t1DM during the period 1978-2008. the mean age at diagnosis was 8.32±5.01 years. the date of birth and the date at diagnosis were recorded from the patients' files. rEsULts: significantly more children were diagnosed with t1DM during the cold months as opposed to the warm months (p=0.001), with no differences between boys and girls. When children were categorized into the age groups ≤3 and >3 years old the seasonal variation pattern was different in younger ages suggesting that environmental factors which possibly interfere with t1DM diagnosis may have a different effect in those of younger than older age. With regard to date of birth, significantly more children with diabetes were born during the spring-summer than in Autumn-Winter (p=0.004). cONcLUsIONs: the results of the study support the concept of seasonality in t1DM diagnosis, implying a possible relationship between clinical expression of t1DM and various climatic factors. seasonal variation at diagnosis appears to be different in younger compared to older children.