This study was carried out to determine whether a short-term zinc supplementation contributes to beneficial changes in glycemic control among type 2 diabetic patients. Seventy-six diabetic subjects and 72 normal adults participated in this study. Subjects were divided into supplemented and control groups. Forty-four diabetic patients and 34 normal subjects were supplemented with 50 mg zinc daily as zinc gluconate for 4 weeks. Zinc status was assessed from fasting plasma levels and urinary excretion. The effects of zinc supplementation on fasting blood glucose, HbA_1c, insulin, and C-peptide were measured at the beginning of the study and after 4 weeks of supplementation. The changes in glycemic control indicators were compared between diabetic groups, classified by baseline HbA_1c levels, and by diabetic duration. At baseline, the incidence of marginal zinc deficiency in the diabetic group, as determined by plasma zinc level, was approximately twice as high as in the normal adult group. The changes of HbA_1c concentration, and fasting blood glucose following supplementation were not statistically significant in diabetic subjects. In normal subjects, a significant decrease of HbA_1c occurred only in the zinc supplemented group. No significant changes were observed for serum insulin and C-peptide in diabetic as well as normal subjects. However, when the changes were compared by baseline HbA_1c level, we found that diabetic subjects with HbA_1c ≥ 7.5% showed significantly improved levels of HbA_1c and fasting glucose after Zn supplementation. While such improvement in fasting blood glucose was significant among diabetics with shorter diabetic duration, significant levels of increase in serum insulin and C-peptide were observed in zinc supplemented subjects with longer diabetic duration. Fasting blood glucose was significantly decreased, whereas serum insulin and C-peptide were increased in diabetics with marginal zinc status. Therefore, we suggest that Zn supplementation for a short-term period may improve glycemic control in diabetic patients with higher HbA_1c levels and marginal zinc status.