Sera from essentially all Swedish children aged 0-14 years with Type 1 (insulin-dependent) diabetes mellitus with onset during an autumn period (October-December 1985) and a late spring period (May-June 1986) were selected. In all, 98 patients were analysed for IgM antibodies against coxsackie B virus serotypes 1 through 5 by a mu-antibody capture radio immunoassay technique. Sera from 94 referent children matched for age, sex and residential area, collected during the same period, were also analysed. During the autumn period, 10 out of 67 (15%) diabetic children were IgM positive while 14 out of 75 (19%) of the healthy referent children demonstrated positivity. During the late spring period only one out of 31 (3%) children with diabetes and two out of 19 (10%) referent children were IgM positive. In the diabetic patients, five were coxsackie B2 positive while coxsackie B1, 3, 4 and 5 were represented by one to three patients each. Eight referent children were coxsackie B4 positive, six were B3 positive and two B2 positive, while no referent children were positive against coxsackie B1 and 5. During these two periods in late 1985 and early 1986 these data demonstrate no evidence of increased antibody frequency against coxsackie B virus 1 through 5 at the onset of childhood diabetes in Sweden.