Islet cell antibodies (ICA) and thyroid microsomal antibodies (MCHA) were studied in many type 1 diabetics. The incidence of ICA and MCHA was 28.8% and 14.4%, respectively. It was significantly higher than in controls (p<0.01). The highest percentage (63.6%) of ICA-positive cases was found during the first year. This incidence gradually decreased as the clinical course proceeded. With regard to the incidence of MCHA, it was also high during the first year but did not decline significantly at 6-10 years after diagnosis. On the other hand, six cases had both ICA and MCHA. Three of these were of short duration and two were of long duration following the onset.