The Zoladex In Pre-menopausal Patients (ZIPP) study was designed to determine whether addition of goserelin ('Zoladex') and/or tamoxifen to adjuvant therapy (radiotherapy and/or chemotherapy), provided benefit to pre- or peri-menopausal women with operable, early breast cancer. A combined analysis of four randomised trials using a core protocol was performed. Patients (n = 2710) were randomised into a 2 x 2 factorial trial based on goserelin and tamoxifen (n = 1800) or randomised to receive goserelin or not (n = 910; some received elective tamoxifen) for 2 years. The analysis presented here compares women who did (n = 1354) or did not (n = 1356) receive goserelin. After a median follow-up of 5.5 years, goserelin provided a significant benefit for event-free survival (hazard ratio [HR] 0.80; 95% confidence interval [CI] 0.69, 0.92; P = 0.002) and overall survival (HR 0.81; 95% CI 0.67, 0.99; P = 0.038). Goserelin was well tolerated. These data show that the addition of goserelin to standard adjuvant therapy is more effective than standard therapy alone in pre-menopausal women with early breast cancer.