We investigated seasonal variation in serum lipids in 96 volunteers aged 65–74 years who were studied at 2-monthly intervals for one year. Periodic regression analysis revealed highly significant seasonal variation in serum total cholesterol and high-density lipoprotein (HDL) cholesterol. Peak levels for both occurred in winter with corresponding summer troughs. The seasonal difference for total cholesterol was 0.32 mmol/1 (95%CI 0.23–0.41, p1 (95%CI 0.12–0.19, p1, 95%CI 0.15–0.39, p< 0.0001), and triglycendes were significantly greater in late winter for women only (seasonal difference 0.22 mmol/1, 95%CI 0.09–0.35, p = 0.002). The timing of seasonal variation in total cholesterol and triglycerides would be consistent with a role in the seasonal variation in vascular deaths, but our finding of a relatively high HDL: total cholesterol ratio in winter makes this less likely.