Ovarian ablation improves survival in premenopausal early breast cancer, but the potential added value by luteinizing hormone-releasing hormone (LHRH) agonists to tamoxifen is still not clear. The purpose of our study is to examine the efficacy of the LHRH agonist goserelin for adjuvant therapy of premenopausal breast cancer, the role of interaction between goserelin and tamoxifen and the impact of estrogen receptor (ER) content. A total of 927 patients were included in the Stockholm part of the Zoladex in Premenopausal Patients (ZIPP) trial. They were randomly allocated in a 2 × 2 factorial study design to goserelin, tamoxifen, the combination of goserelin and tamoxifen or no endocrine therapy for 2 years, with or without chemotherapy. This is formally not a preplanned subset analysis presenting the end point first event. In this Stockholm sub-study, at a median follow-up of 12.3 years, goserelin reduced the risk of first event by 32% (P = 0.005) in the absence of tamoxifen, and tamoxifen reduced the risk by 27% (P = 0.018) in the absence of goserelin. The combined goserelin and tamoxifen treatment reduced the risk by 24% (P = 0.021) compared with no endocrine treatment. In highly ER-positive tumours, there were 29% fewer events among goserelin treated (P = 0.044) and a trend towards greater risk reduction depending on the level of ER content. The greatest risk reduction from goserelin treatment was observed among those not receiving tamoxifen (HR: 0.52, P = 0.007). In conclusion, goserelin as well as tamoxifen reduces the risk of recurrence in endocrine responsive premenopausal breast cancer. Women with strongly ER-positive tumours may benefit more from goserelin treatment. The combination of goserelin and tamoxifen is not superior to either modality alone. With the limitations of a subset trial, these data have to be interpreted cautiously.