ObjectiveTo determine whether there is a relationship between season of birth and adult blood pressure, as a possible early determinant of later blood pressure.DesignWe studied 572 men, aged 45–64 years, whose blood pressure was measured in standardized manner as part of a nation-wide survey in Spain. To analyse the seasonal variation in blood pressure, a linear regression was performed, adjusting for age, height, body mass index, occupation and rural or urban residence.ResultsWe found seasonal variation in mean systolic blood pressure, with maxima in adults born in autumn and winter, and minima in those born in spring and summer. The greatest difference in systolic blood pressure occurred between adults born in spring (134.1 mmHg) and those born in autumn (140.3 mmHg). After adjustment, the difference in means between spring and autumn was 5.9 mmHg (95% confidence interval 0.7 to 11.1 mmHg,P= 0.03).ConclusionsThis study demonstrated differences in systolic blood pressures of adult men according to the Jose season of their birth. Although this relationship is compatible with several hypotheses, the difference found between spring and autumn, partially independent of some other factors, might indicate that the extent of early exposure to sunlight is implicated in determining later blood pressure. This needs further investigation.