Background: Suppressing ovarian function of women with breast cancer may improve outcome by preventing estrogenic stimulation of any residual cancer, particularly for pre-menopausal women with estrogen receptor (ER)-positive tumors. We report a collaborative meta-analysis of individual participant data from randomized trials of ovarian ablation or suppression. Methods: Data were sought from randomized trials that compared ovarian ablation or suppression versus not. Primary analyses included only premenopausal women age < 55 with ER-positive or unknown tumors, stratified into those who received no chemotherapy, or remained premenopausal following chemotherapy, and those whose menopausal status following chemotherapy was not ascertained. Standard log-rank methods estimated ER-weighted annual event rate ratios (RR). Results: Individual patient data were provided for 25 of 27 relevant trials, comprising 14,993 (98.7%) of 15,195 women randomized. Overall, fewer breast cancer recurrences were seen with ovarian ablation/suppression than control (RR = 0·82, 95%CI 0·77–0·88; p < 0·0001). Recurrence reductions were significantly (p = 0.0003) larger among women (n = 7,213) known to be premenopausal prior to ovarian suppression (RR = 0·70, 0·63–0·78; p = 0·0003) than among those (n = 7,786) whose menopausal status was uncertain after chemotherapy (RR = 0·91, 0·83–0·99; p = 0·03). For known premenopausal women, 15-year risk of recurrence was improved by 12·1% (28·9% vs 41·0%; p < 0·0001. 15-year breast cancer and all-cause mortality were improved by 8·0% (20·9% vs 28·9%; RR 0·69, 0·60–0·80; p < 0·0001) and 7.2% (26·0% vs 33·1%; RR = 0·73, 0·64–0·82; p < 0·0001), respectively, with no increase in deaths without recurrence (RR = 0·88, 0·67–1·14; p = 0·33). Recurrence reductions were significantly (p = 0·003) larger among premenopausal women aged under 45 (RR = 0·63, 0·55–0·72; p < 0·0001) than among those aged 45-54 (RR = 0·84, 0·70–1.00; p = 0·045), but did not differ significantly by other recorded patient or tumor characteristics. Conclusions: For pre-menopausal women aged under 45, ovarian ablation or suppression substantially reduces the 15-year risk of recurrence and death from breast cancer without increasing mortality from other causes.