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Increased mortality from cardiovascular disease in winter is partly explained by increased blood pressure (BP) caused by cold exposure. For physicians, instruction in home heating is feasible option to reduce cold exposure, but the effectiveness remains unknown.To determine whether instruction in home heating increases indoor temperatures and decreases ambulatory BP among elderly people, we conducted an open-label, simply randomized, controlled trial in the winters.As an intervention, the participants were asked to set the heating device in the living room to start 1 h before estimated rising time with target temperature at 24°C, and to stay in the living room until 2 h after rising as long as possible. Repeatedly measured ambulatory BP, physical activity, and indoor temperatures until 4 h after rising were assessed using multilevel linear regression model with random intercept among individual.A total of 359 eligible participants (mean age ± standard deviation: 71.6 ± 6.6) were randomly allocated to the control group (n = 173) and intervention group (n = 186). Intervention significantly increased living room temperature by 2.09°C (95% confidence interval 1.28–2.90), and significantly decreased SBP and DBP by 4.43/2.33 mmHg (95% confidence interval 0.97–7.88/0.08–4.58 mmHg) after adjusting for confounders including age, sex, antihypertensive medication, household income, and physical activity.Short-term effect of instruction in home heating showed larger increase of indoor temperature than that of insulation intervention. Significant reduction of BPsuggests the effectiveness on preventing cardiovascular incidence in winter.To summarize, instruction in heating significantly decreased BP.