It has been postulated that immune modulation and activation play an important role in the pathogenesis of type 2 diabetes mellitus (T2DM), but evidence for this has not yet been well documented. We explored the changes in peripheral immunocompetent cells in relationship to the severity of T2DM in 142 patients, and 34 healthy individuals in Japan. A severity index with 0-12 grades was derived based on the HbA1c level and the number of complications. By multiple regression analysis, the severity index was positively associated with neutrophil counts and negatively associated with platelet and CD19+ lymphocyte counts. However, we did not observe any significant changes in other lymphocyte subsets such as CD4+, CD8+, and CD56+. These results suggest that poor diabetic control may be marked by changes in some blood cell types.