Previous studies have shown that both exogenous and endogenous estrogens and progestogens increase the risk of breast cancer. it has been suggested that in vitro fertilization (IVF) procedures may increase breast cancer risk. This risk could be could be due to a temporary decrease in estradiol and progesterone levels (during down-regulation of the natural cycle), as well as strongly elevated levels (during stimulation phase). Results of previous studies investigating breast cancer risk after IVF treatment were inconclusive because of limited follow-up. Recent studies have reported no increased risk of this cancer after IVT at a mean follow-up of 8 and 16 years. This historical cohort (OMEGA) study quantified long-term risk of breast cancer after ovarian stimulation for IVF. The aim of the study was to compare the long-term risk of breast cancer among a nationwide cohort of women receiving ovarian stimulation for IVF with that of women receiving other fertility treatments and those in the general population. The study was conducted in 12 IVF clinics in the Netherlands. The cohort was composed of 19,158 women who started IVF treatment between 1983 and 1995 (IVF group) and 5950 women who started other fertility treatments between 1980 and 1995 (non-IVF group) Data were obtained from all 12 IVF clinics. There was complete follow-up through December 2013 for 96% of the cohort. At end of follow-up, the median age was 53.8 years in the IVF group and 55.3 years in the non-IVF group. Information on ovarian stimulation for IVF, other fertility treatments, and potential confounders was obtained from medical records and mailed questionnaires. First invasive and in situ breast cancer incidence among women who underwent fertility treatments was obtained through linkage with the Netherlands Cancer Registry (1989-2013). Risk of breast cancer in the IVF group was compared with risk in the non-IVF group (hazard ratios [HRs]) and the general population (standardized incidence ratios [SIRs]). In the total cohort, the mean age at baseline was 32.8 years, and the mean number of IVF cycles was 3.6. After a median follow-up of 21.1 years, 839 of the cohort had invasive breast cancer, and 109 had in situ breast cancer. Compared with the general population, breast cancer risks were not increased either in the IVF group (SIR, 1.01; 95% confidence interval [CI], 0.93-1.09) or the non-IVF group (SIR, 1.00; 95% CI, 0.88-1.15). At the age of 55 years, cumulative incidence rates of breast cancer were 3.0% in the IVF group and 2.9% in the non-IVF group (P = 0.85). With longer time since treatment (>= 20 years), the SIR did not increase in the IVF group (SIR, 0.92; 95% CI, 0.73-1.15) or in the non-IVF group (SIR, 1.03; 95% CI, 0.82-1.29). Women with 7 or more IVF cycles had a significantly decreased risk of breast cancer than did women who were treated with 1 to 2 IVF cycles; the HR was 0.55, with a 95% CI of 0.39 to 0.77. The risk was also significantly lower (HR, 0.77; 95% CI, 0.61-0.96) after poor response to the first IVF cycle (<4 collected oocytes) versus normal response (>= 4 collected oocytes). These data show that IVT treatment did not increase the risk of breast cancer in a cohort of women undergoing fertility treatment in the Netherlands between 1980 and 1995 when compared with women who received non-IVF treatment or those in the general population after a median follow-up of 21 years. These findings are consistent with those from recent reviews showing no significant increase in long-term risk of breast cancer among IVF-treated women.