['The magnitude of thrombotic risk during ovarian stimulation cycles is not known. We calculated the magnitude of thrombotic risk in a cohort of women starting a new cycle of ovarian stimulation and investigated the role of inherited and acquired thrombophilia for these events.', 'This is an observational study involving outpatients of a clinical research centre. Consecutive women undergoing ovarian stimulation (n = 305) were enrolled. Blood samples for studying inherited and acquired thrombophilia were obtained > or = 2 months after the last cycle of treatment. Odds ratios (OR) and confidence intervals (CI) were determined for markers significantly associated with thrombotic events. Blood samples were analysed for inherited and acquired causes of thrombophilia (antithrombin, protein C, protein S, antiphospholipid antibodies, the Factor V Leiden and FIIA20210 mutations, the TT677 MTHFR genotype, and homocysteine plasma levels).', 'Thrombotic events were observed in 4/747 cycles of ovarian stimulation, with a prevalence of 0.5%, corresponding to 1.6 per 100 000 cycles/woman. Age > or = 39 years and homocysteine plasma levels above the 97.5 percentile were significantly associated with thrombotic events during IVF [OR 15.2 (95% CI 2.0-115.0) and 14.4 (1.5-141.3) respectively].'] Are age and homocysteine plasma levels risk factors for thrombotic complications after ovarian stimulation?