A comprehensive survey of ketosis-prone diabetes showed a 29% preponderance of males and a trend for the time of diagnosis to coincide with epidemics of viral diseases. There was a significantly increased incidence between July and January in the age group diagnosed at 0-14 years but this seasonal trend was not shown in a group of boys diagnosed at this age who were all born in the month of October. The incidence of diabetes in this group of boys was very much higher than would be expected (p less than 0.00001). HLA histocompatibility types and virus infections were considered as possible causes of this striking finding but there seemed to be at least one additional seasonal factor acting prenatally. Circumstantial evidence suggested that this additional factor is the N-nitroso-compound content of processed mutton traditionally consumed in Iceland in the two weeks from December 23. Recently produced Icelandic smoked/cured mutton contains considerable concentrations of N-nitroso-compounds. This work implies that a common food additive contributes to the production of ketosis-prone diabetes, not in the consumer but in the progeny.