Adjuvant breast cancer therapy with tamoxifen is associated with greater disease-free survival and possibly overall survival. Long-term treatment, possibly of indefinite duration, is being evaluated. Compliance with long-term therapy will depend largely on the nature and severity of tamoxifen's side effects. We evaluated the symptoms associated with tamoxifen therapy in 140 postmenopausal women with axillary node negative breast cancer in remission (mean years since menopause, 9.3) enrolled in a placebo-controlled, randomized toxicity study. Tamoxifen recipients reported moderate or severe vasomotor symptoms up to 17%, and gynecologic symptoms up to 4% more frequently than placebo subjects. Persistent vasomotor, gynecologic, or other major side effects were reported by 48% of tamoxifen recipients, and by 21% of placebo subjects. These carefully collected data suggest significant perceived symptom 'costs' of tamoxifen therapy in postmenopausal women, of a magnitude likely to compromise long-term compliance. (Arch Intern Med.1991;151:1842-1847)