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To quantify the association between systolic blood pressure and season, indoor and outdoor temperature and short-term trends in outdoor temperature.The study used data from the WHO MONICA Project risk factors surveys from 25 populations in 16 countries. Random samples of men and women aged 35–64 years were invited to participate. Systolic blood pressure measurements were available for 115 434 participants. Hierarchical models were used to quantify the association between blood pressure and temperature, and account for differences in the associations between populations.Populations closer to the equator showed larger seasonal changes in blood pressure. A 1°C increase in indoor temperature reduced systolic blood pressure by an average of 0.31 mmHg (95% posterior interval: −0.44, −0.19). A 1°C increase in outdoor temperature reduced blood pressure by the smaller average of 0.19 mmHg (95% posterior interval: −0.26, −0.11). Increased outdoor, but not indoor, temperatures had a stronger effect in women than in men. The effect of outdoor temperature remained after controlling for indoor temperature. Short-term trends in temperature did not have a statistically signicant effect.Indoor and outdoor temperature have independent effects on systolic blood pressure, and both should be controlled for in studies that measure blood pressure. Improved protection against cold temperatures could lead to a reduction in the winter excess of cardiovascular mortality.