Objective: The aim of this study on primary breast cancer patients undergoing adjuvant tamoxifen treatment was to determine how their perceptions of the treatment and their experience of side-effects contributed to their adherence to the treatment. Methods: A consecutive series of primary breast cancer patients eligible for tamoxifen therapy were studied qualitatively by conducting semi-structured in-depth interviews at two French cancer centres. Results: The women aged 35-65 (N 5 34) were struggling with several issues involving their understanding and experience of the treatment, which have not been documented so far. These issues included confusion about the 'hormonal' nature and activity of tamoxifen and the etiology of the changes in their menopausal status, as well as the symbolic associations formed by patients about the paradox of taking a treatment that has aging effects but saves lives. Conclusions: This study shows the great physical burden often associated with tamoxifen treatment and brings to light women's own complex representations of the treatment and their interpretation of the side-effects. Better communication between health-care providers and patients should ultimately help to prevent refusal or discontinuation of tamoxifen treatment.