A distinct seasonal incidence pattern in the onset of detection of diabetes mellitus among school-age children is apparent. However, the true picture of seasonal variation in the onset of symptoms and detection of juvenile diabetes mellitus in this country may have to await the establishment of regional or national diabetes registries similar to those in Great Britain and Eire. Such data when available must be interpreted with caution so that aggravating, precipitating, and true etiological factors underlying seasonal variation in the onset of juvenile diabetes mellitus are distinguished.