Authors have continued clinical research on type 2 diabetes mellitus (T2DM). The trigger of this research was to make notice of elevated HbA1c of younger male diabetics for hot climate in the summer 2018. Enrolled subjects were 89 male patients with T2DM. Methods include the classification of 6 groups by the age, which are 21-40, 41-50, 51-60, 61-70, 71-80 and 81-90. HbA1c values in median were calculated for five seasons of 15 months. Basal HbA1c in 6 groups was 7.0%, 7.1%, 7.2%, 7.2%, 6.9% and 7.0%, respectively. Seasonal changes in HbA1c values are as follows: i) groups 21-50 showed highest in the summer, ii) groups 51-70 showed gradually decrease from winter to summer, iii) groups 71-90 showed gradually decrease from winter to autumn, and increase for winter. For seasonal HbA1c changes, influence of hot climate during from spring to summer may be involved for 21-50 years. More activity in spring to summer may be related for 51-70 years. Less exercise and more eating may be observed for 71-90 years. There are not enough analyze for related factors, then further study concerning various biomarkers would be expected
