Mortality from ischaemic heart disease (IHD) in northern Finland was examined on a regional basis, correlating that observed in the general male population with environmental temperature and individual risk factors (smoking, serum lipids and blood pressure) as available from a health survey of 571 rural men. The age-adjusted mortality from IHD increased by 5% (95 C.I. 1-9%) per 1 degree C decline in January mean temperature, but this finding was largely due to high mortality in a single commune (Kittilä). The probability that a subject was living in this high mortality area increased by a factor of 16 (95% C.I. 4-58) per 1 degree C decline in temperature and was 5-fold in smokers as compared with non-smokers. The findings conform with previous reports claiming that climate is one factor contributing to regional differences in IHD.