BACKGROUND: Lifetime estrogen exposure has been related to breast cancer risk, osteoporosis, and cardiovascular disease but data on venous thromboembolism (VTE) risk are limited. METHODS: Data from a hospital-based case-control study among 608 postmenopausal women (191 with a first episode of idiopathic VTE and 417 age-matched controls) were used to determine whether estrogen exposure, as assessed by age at menopause [classified as early (< or = 45 years), normal (46-54 years) and late menopause (> or = 55 years)] and parity, was associated with the risk of VTE. RESULTS: After adjustment for potential confounding variables, the risk of VTE was increased with each year's delay in the menopause [odds ratio (OR) = 1.06, 95% confidence interval (CI) = 1.02-1.10, P < 0.0075]. When compared with women with normal menopause used as a reference, the adjusted OR for VTE was 0.59 (95% CI = 0.36-0.97) and 2.53 (95% CI = 1.28-4.99) for women with early menopause and late menopause, respectively (P = 0.001). Adjusted OR for VTE was also higher for women with more than two children when compared with those with less than or equal to two children (1.56, 95% CI = 1.03-2.34, P = 0.03). The lowest risk of VTE was observed in women with early menopause and lower parity (adjusted OR = 0.60, 95% CI = 0.30-1.24), the highest risk was among women with late menopause who have had more than two children (adjusted OR = 3.41, 95% CI = 1.46-9.25). CONCLUSION: These results show that the longer exposure to endogenous estrogen is associated with an increased VTE risk.