Although low birthweight has been related to higher mean blood pressure in individuals, there have been very few studies on the contribution of the 'fetal origins hypothesis' to international variations in mean blood pressure. Standardized data on mean blood pressure levels in adults aged 20-29 years (available from the INTERSALT study countries) were related to data on mean birthweight and infant mortality rates (IMRs) approximately 25 years earlier (available from the World Health Organization and the United Nations statistics for 24 of these countries). Population mean systolic pressure was inversely correlated with IMR (r = -0.66, P < 0.001) and positively correlated with mean birthweight (r = 0.36, P = 0.108). These correlations were little affected by adjustment either for current body mass index or for current sodium, potassium, and alcohol intake. The directions of these associations suggest that low mean birthweight and high infant mortality are not important determinants of high population mean adult blood pressure levels.