Improved survival of premenopausal women with breast cancer has prompted requests for hormone treatment to control climacteric symptoms. Observations to date suggest that there is a low risk of worsening the prognosis by giving hormonal replacement therapy (HRT). This report details findings in a Scandinavian study of HRT, the HABITS (Hormonal Replacement Therapy After Breast Cancer-Is It Safe?) trial. An open randomized trial design was used. Participants were to receive either HRT for 2 years or the best treatment not including hormones. Intention-to-treat analysis was carried out on 434 women who were randomized in the study; 345 of them were followed up at least once. The 2 groups were similar at the outset. After a median follow-up interval of 2.1 years, 26 women in the HRT group and 8 in the nonHRT group had had a new breast cancer event. Eleven events in the HRT group were local recurrences, 5 were contralateral cancers, and 10 were distant metastases. The corresponding numbers for the control group were 2, 1, and 5. In addition to the HRT group, 2 women assigned to the control group were exposed to HRT. In most cases, the event took place during ongoing treatment. The form of HRT (continuous combined, sequential, estrogen only) did not appear to be a significant factor. Three women in the HRT group and 4 in the nonHRT group died of breast cancer. In addition to breast cancer, 1 woman in the HRT group developed lung cancer and another endometrial cancer. Two women in the control group had progression of breast cancer. The HABITS trial was ended prematurely because of an unacceptably high risk of breast cancer in women with a history of breast cancer who received HRT. Actively treated women have been advised to discontinue HRT and to take the most effective nonhormonal medication. At least 5 additional years of follow up are planned.