In 150 patients (aged 4-16) with type I diabetes, hospitalized in the Paediatric Institute in Lublin in the years 1968 bis 1979, some epidemiological data were analysed, including age, sex and the month of onset of diabetes. An increase of the number of onsets of diabetes type I was observed in the second half of the year, particularly in September and October, similarly to the occurrence of some virological infections, including Coxsackie B-4. Our question was if the Coxsackie B-4 virus could have triggered the onset of diabetes. In 66 children (2,5-14 years old) with recent onset of diabetes, the neutralising antibody titer to Coxsackie B-4 virus was determined. We found it possible to determine the titres of the antibodies in 34 of these patients 3-5 years after the first hospitalization. Titres were also determined, during the same season, in a control group consisting of 42 persons. Patients with recent onsets of type I diabetes had significantly higher titres than in the control group, namely P-0.01 titres 1:64-1:256 constituting 42.4% while the control group showed 14.3%. During the 3-5 year course of diabetes the titres decreased to the levels observed in the control group. Our observations confirmed the participation of Coxsakie B-4 virus in the evocation of IDDM.