Age, sex, and estimated time of onset of insulin-dependent diabetes were determined for children in Pittsburgh (N = 673), Gainesville (N = 976), Galveston (n = 741), and Melbourne (N = 851). The US cities had a decrease in new cases during the summer and peak incidence in January through April. In Melbourne, monthly trends were reversed: there were more cases during May through August. In US cities, but not in Melbourne, children less than 6 years old showed a greater variation by season than children 6 years old and older. Observations of the same fall and winter onset (in different calendar months) of insulin-dependent diabetes in Australia and the United States, and exaggeration of seasonal differences in young US children, suggest that onset of insulin-dependent diabetes is associated with seasonally varying viral diseases. Mumps and rubella infections do not seem to be responsible for much of the seasonal variation. Seasonal peaks of mumps and rubella are later than those observed for insulin-dependent diabetes, and immunization with live mumps and rubella viruses has not been associated with changes in incidence of insulin-dependent diabetes. An increase in disease incidence in boys over girls below age 6 years and in girls over boys at ages 6 through 11 years was consistently observed but not explained.