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 (pEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)0.001), with no differences between boys and girls. When children were categorized into the age groups 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 (pEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)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.