AbstractWe studied ICA, HLA and insulin secretory capacity in 87 children with positive urinary screening and more than 2 points in the oral glucose tolerance test in order to establish criteria by which they could be classified into type 1 or type 2 diabetes mellitus. Fifty‐five non‐obese, ketosis‐prone insulin dependent diabetic children were used as controls for type 1 diabetes mellitus. Our conclusions were as follows:1. Type 1 diabetics were non‐obese (on insulin therapy), ICA positive, ketosis‐prone, had an insulin secretory capacity (Z IRI) of less than 100/nU/ml, and most of them possessed HLA‐Bw54‐DR4 or DRw9, DRw53 but did not possess Bw52‐DR2 haplotype.2. In the patients who were treated by dietary regimens alone for certain periods, however, insulin secretory capacities gradually deteriorated and they finally became insulin dependent. The children of this group who were not obese during insulin therapy and possessed an HLA haplotype identical to that in type 1 diabetes, regardless of ICA, might be classified as having slowly progressive type 1 diabetes.3. The major difference between type 1 and slowly progressive type 1 diabetes was a family history of diabetes. Genetic factors might modify the clinical course of type 1 diabetes mellitus.4. If the sensitivity of ICA or related autoantibodies to islet cells can be detected more readily, it should become easier to distinguish between type 1