Background : The Breast Cancer Prevention Trial (BCPT) is a large, multicenter chemoprevention trial testing the efficacy of the antiestrogen drug tamoxifen for prevention of breast cancer and coronary heart disease in healthy women at high risk of breast cancer. The BCPT evolved from a series of prior studies in early stage breast cancer demonstrating the efficacy of tamoxifen in the prevention of systemic breast cancer recurrence and in the reduction of contralateral breast cancers. Purpose: The purpose of this article is to describe the methodologic considerations in the collection of health-related quality-of-life (HRQL) data in the BCPT and to present base-line HRQL data on the first 9749 participants. Methods: An HRQL questionnaire that included the Center for Epidemiologic Studies-Depression Scale, a symptom checklist, the Medical Outcomes Study 36-item short form (MOS-SF-36), and the MOS sexual problems questions was completed by participants in the BCPT at base line (prior to random assignment). Medical and demographic information, as well as projected risk of breast cancer, were collected as part of study eligibility. Descriptive and correlational data were examined for these study participants. Results: BCPT participants report high levels of functioning compared with U.S. general population norms but still report an average of 8.9 distinct symptoms during the past 4 weeks. Depression is less prevalent among the participants than in community samples, which reflects the exclusion of clinically depressed individuals. Sixty-five percent reported being sexually active in the past 6 months, with an age-related decline in sexual activity. Younger women reported fewer sexual problems than older women. There is a strong correlation between the two mental health measures, moderate to weak correlations between HRQL scales and levels of self-reported symptoms, and only weak correlations between measures of breast cancer risk and HRQL scales. The MOS-SF-36 scores were examined for three consecutive recruitment samples (0-6 months, 7-12 months, and 13-20 months), and the base-line scores were slightly better for the earliest group of participants. Conclusions: This article demonstrates the feasibility of collecting HRQL data in a large, multicenter, chemoprevention trial for women at high risk of breast cancer. The successful integration of HRQL data collection into this clinical trial attests to its value as a safety-monitoring end point and as an explicit and measurable outcome for the entire trial. Implications: HRQL data are important for studies in which healthy populations are involved and in which the potential for decrements in quality of life are real or perceived. [J Natl Cancer Inst 1995;87:1372-82]