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Aim: The aim of this study was to investigate whether tamoxifen toxicity and treatment discontinuations differed in the adjuvant versus chemopreventive setting., Methods: At our Institutions 119 postmenopausal breast cancer patients were randomized from August 1987 to March 1995 to tamoxifen only within adjuvant studies (International Breast Cancer Study Group studies VII and IX) and 202 healthy hysterectomized women aged 35-70 years were randomized from November 1993 to May 1996 in a multicenter, double-blind, placebo-controlled chemoprevention study (Italian Tamoxifen Prevention Study). The tamoxifen dose was 20 mg/day for 5 years in all studies. Median age was 66 years (54-85) in the adjuvant studies and 53 years (37-69) in the chemoprevention study. Median treatment duration was 238 and 120 weeks, respectively., Results: Patients treated within adjuvant studies experienced more hot flashes, vaginal discharge and/or bleeding, bone marrow depression and weight gain than those treated in the chemoprevention study, consistent with the fact that a proportion of women in the latter study were receiving placebo. Temporary discontinuation occurred in 2.5% of patients in the adjuvant studies and in 5.4% of women in the chemoprevention study (difference not statistically significant). Permanent discontinuation was more frequent in the chemoprevention study than in the adjuvant ones (26.7% vs 15.1%--P< 0.05)., Conclusions: In summary, our data show that, although the toxicity of tamoxifen is superimposable in the two settings, a larger proportion of women treated as chemoprevention discontinue treatment spontaneously. Due to the double-blind nature of the chemoprevention study, the impact of the toxicity of tamoxifen upon compliance in the chemopreventive setting cannot be ascertained.