The aim of the study was to determine the frequency of patients with gestational diabetes mellitus (GDM) who have serological markers typical of autoimmune type 1 DM. The specific pancreatic markers, ICAs, glutamic decarboxylase (GADAbs), and second islet antigen (IA2Abs), were measured in 70 women with GDM during the pregnancy and after delivery. ICAs were measured by indirect immunofluorescence and GADAbs and IA2Abs were determined by a radiobinding assay with recombinant antigens. On entering the study, 1 of 70 (1.4%) patients was positive for both ICAs (80 JDF-U) and GADAbs (167 U/mL), while another (1.4%) was positive for ICAs (40 JDF-U). None of the patients was positive for IA2Abs. During follow-up, positivity was maintained unchanged in the two positive patients. Four previously negative patients had seroconversion: one for both ICAs (20 JDF-U) and GADAbs (49.3 U/mL) and the other three for GADAbs (1.8, 1.4, and 15.3 U/mL, respectively). The IA2Abs remained negative in all patients. Overall, during the observation period 6 of 70 (8.6%) patients had or developed autoantibodies against endocrine pancreas. During follow-up 15 patients developed clinical DM (10 type 2, 5 type 1) and 7 demonstrated impaired glucose tolerance (IGT) after OGTT. No correlations were demonstrated between the immunological patterns and the evolution in DM. In patients with GDM, the frequency of pancreatic autoantibodies varies during the pregnancy and after delivery, but a small subgroup of patients bearing these markers is identifiable. GDM is a complex syndrome, constituted by different types of diabetes mellitus where the autoimmune form is very rare.