Neonatal insulin-dependent diabetes mellitus (IDDM) occurs rarely. A sibship of two HLA-Dw3/4-positive boys who developed IDDM within the 1st wk of life is described. Although the HLA-D region genotype would be consistent with IDDM associated with islet autoimmunity, islet cell antibodies were negative, but both boys exhibited the presence of a novel autoantibody that reacted specifically with a conspicuous, yet unidentified, determinant in the interstitial tissue among the acinar cells. The possible relationship between this acinar nonislet autoantibody and permanent neonatal diabetes remains to be established.