OBJECTIVE: To determine the magnitude and importance of the relation between smoking, bone mineral density, and risk of hip fracture according to age. DESIGN: Meta-analysis of 29 published cross sectional studies reporting the difference in bone density in 2156 smokers and 9705 non-smokers according to age, and of 19 cohort and case-control studies recording 3889 hip fractures reporting risk in smokers relative to non-smokers. RESULTS: In premenopausal women bone density was similar in smokers and non-smokers. Postmenopausal bone loss was greater in current smokers than non-smokers, bone density diminishing by about an additional 2% for every 10 year increase in age, with a difference of 6% at age 80. In current smokers relative to non-smokers the risk of hip fracture was similar at age 50 but greater thereafter by an estimated 17% at age 60, 41% at 70, 71% at 80, and 108% at 90. These estimates of relative risk by age, derived directly from a regression analysis of the studies of smoking and hip fracture, were close to estimates using the difference in bone density between smokers and non-smokers and the association between bone density and risk of hip fracture. The estimated cumulative risk of hip fracture in women in England was 19% in smokers and 12% in non-smokers to age 85; 37% and 22% to age 90. Among all women, one hip fracture in eight is attributable to smoking. Limited data in men suggest a similar proportionate effect of smoking as in women. The association was not explained by smokers being thinner, younger at menopause, and exercising less nor by actions of smoking on oestrogen, but smoking may have a direct action on bone. CONCLUSIONS: Hip fracture in old age is a major adverse effect of smoking after the menopause. The cumulative excess bone loss over decades is substantial, increasing the lifetime risk of hip fracture by about half.