In this study, a group of controls and patients with essential hypertension were followed up for 1 year. Four measurements at different day temperatures were performed. In each visit, blood pressure, serum total cholesterol, and high-density lipoprotein cholesterol (HDL-C) were measured. The results showed a significant inverse relationship between mean blood pressure and serum total cholesterol levels with day temperature, while a direct relationship was observed for HDL-C value. These results suggest that in areas where significant changes in day temperature and daylight duration exist at different times of the year, blood pressure, serum cholesterol, and HDL-C levels change accordingly in a cycle with higher blood pressure and serum total cholesterol and lower HDL-C values in the coldest season.