An east-west regional gradient in cardiovascular mortality was found within seven counties in mid-Sweden during the years 1969-1983. The mortality differences were of considerable magnitude for ischaemic heart disease (IHD) as well as for stroke. In previous reports, in which the distribution of risk factors among middle-aged men was presented, the moderate variation among the communities could not explain the mortality variation. Water hardness has previously been reported to be inversely related to cardiovascular mortality in several countries. In this paper, water samples from all 76 communities in seven counties were analysed in relation to mortality rates from IHD and stroke for men and women. Water hardness (Ca+Mg and other minor constituents), and the sulphate and bicarbonate concentrations of the drinking water were inversely related to IHD as well as stroke mortality. The water factors were also inversely related to non-fatal IHD even when account was taken of the age variation and the traditional risk factors as measured by a postal questionnaire. Variation of the water factors accounted for 41% of the variation in IHD mortality rate and 14% of the variation in stroke mortality rate over the 76 communities.