Type 1 and type 2 diabetes frequently co-occur in the same families, suggesting common genetic susceptibility. Such mixed family history is associated with an intermediate phenotype of diabetes: insulin resistance and cardiovascular complications in type 1 diabetic patients and lower BMI and less cardiovascular complications as well as lower C-peptide concentrations in type 2 diabetic patients. GAD antibody positivity is more common in type 2 diabetic patients from mixed families than from common type 2 diabetes families. The mixed family history is associated with more type 1–like genetic (HLA and insulin gene) and phenotypic characteristics in type 2 diabetic patients, especially in the GAD antibody–positive subgroup. Leaving out the extreme ends of diabetes phenotypes, young children progressing rapidly to total insulin deficiency and strongly insulin-resistant subjects mostly with non-Europid ethnic origin, a large proportion of diabetic patients may have both type 1 and type 2 processes contributing to their diabetic phenotype.