We report a rare case of monozygotic (MZ) twins who developed simultaneous onset of type 1 diabetes mellitus (T1DM). Laboratory finding showed similar values of blood sugar, pH, glycosylated hemoglobin, and C-peptide. Urinary sugar and ketones were detected in both. Endocrine and immunological assessment showed similar results. No evidence (clinical or serological) of recent viral or bacterial infection was found. In the 4 years of follow-up, the twins also showed a similar course of disease. Concordance rates for T1DM are high in MZ twins; nevertheless, a simultaneous onset and a similar course of disease are unusual, as well as the HLA allelic variants of our patients. This extraordinary concordance in a pair of MZ twins could be the consequence of a greater environmental similarity or the role of genetic factors other than HLA genes in our twins.