Dense mammographic patterns are a strong predictor of breast cancer risk. Factors at differing stages of life have been linked to breast cancer risk, although rarely studied simultaneously. We aimed to investigate whether birth weight and factors later in life were associated with mammographic density in the Newcastle Thousand Families Study. The Study originally consisted of all 1142 babies born in May and June 1947 to mothers resident in Newcastle upon Tyne in Northern England. Detailed information was collected prospectively during childhood, including birth weight and socio-economic circumstances. At age 49-51 years, 574 study members completed a 'Health and Lifestyle' questionnaire. Of the 307 surviving women who returned these questionnaires, 199 returned a further questionnaire asking for details of routine mammographic screening, their reproductive and contraceptive history. Mammographic patterns were coded into Wolfe categories. This was analysed, by ordinal logistic regression, in relation to a range of variables at different stages of life. Increased standardised birth weight (odds ratio, OR 1.32 (95% CI 1.02-1.71) P = 0.03) was a significant independent predictor of higher density. Increasing body mass index (BMI) was predictive of lower density (OR 0.86 per Kg/m(2) (95% CI 0.81-0.92) P < 0.001), as was having reached menopause (OR, compared to pre- and peri-menopausal, 0.41 (95% CI 0.23-0.73) P = 0.002). Interactions were seen between menopausal status and both BMI and age at menarche (P = 0.06) on density, although for neither did the direction of association change. After adjustment for factors acting throughout life, we identified a significant association between standardised birth weight and density in adulthood, consistent with previous research suggesting that heavier babies have an increased risk of breast cancer in later life. We also confirmed associations between both BMI and menopausal status.