110 people in whom insulin-dependent diabetes developed when they were less than 30 years old were studied as soon as possible after diagnosis. There was evidence for clustering of cases with BW15-positive phenotypes during the winter peak (1976) but not during the autumn peak (1975). Subjects who were BW15-positive, and in particular those who were both B8 and BW15-positive, had higher neutralising antibody titres to Coxsackle virus types B1-B4 58% of cases had islet-cell antibodies (I.C.A.), but the presence of I.C.A. was not correlated with HLA phenotypes or viral antibody titres. In 41 subjects (37%), who gave a definite history of antecedent illness, evidence indicated that this was a precipitating infection and not the initiating event producing islet-cell damage. Nearly half the subjects had had diabetic symptoms for more than 4 weeks before diagnosis.