OBJECTIVE: To assess how effectively measures adopted in extreme cold in Yakutsk control winter mortality. DESIGN: Interviews to assess outdoor clothing and measure indoor temperatures; regressions of these and of delayed cause-specific mortalities on temperature. Setting Yakutsk, east Siberia, Russia. SUBJECTS: All people aged 50-59 and 65-74 years living within 400 km of Yakutsk during 1989-95 and sample of 1002 men and women who agreed to be interviewed. MAIN OUTCOME MEASURES: Daily mortality from all causes and from ischaemic heart, cerebrovascular, and respiratory disease. RESULTS: Mean temperature for October-March 1989-95 was -26.6 degreesC. At 10.2 degrees C people wore 3.30 (95% confidence interval 3.08 to 3.53) layers of clothing outdoors, increasing to 4.39 (4.13 to 4.66; P