We report an open-label, prospective, crossover study involving 184 post-menopausal women experiencing hot flushes on adjuvant tamoxifen (T). Six weeks after switching to an AI, the primary end point, hot flush score, improved by 47.3% (Po0.001) compared to those reported on T. The mean mood rating scale (MRS) score improved by 9.7% (P ¼ 0.01). The total mean combined FACT (b þ es) score improved from 134.2 (95% CI ± 2.96) to 143.5 (95% CI ± 2.96 o0.001), and the endocrine subscale improved by 9.8% from 51.73 (95% CI ±1.38) to 57.34 (CI ±1.38, Po0.001). At 6 weeks, significantly more women chose to remain on an AI: 133 (72%), vs 40 (22%) (Po0.001) preferring T. At 3 months, 107 (58%) preferred to remain on an AI, 55(30%) on T, and 22 (12%) withdrew. The overall arthralgia rate at 3 months was 47% on AI and 30% on T (P ¼ 0.001). In all 182 (99%) women reported appreciating the opportunity to experience both drugs. These data suggest that if patients suffering significant adverse effects on T are given the opportunity to try an AI, this empowers them to prioritise relative side-effects, improving wellbeing in a significant proportion. These data also highlight the need for hospital follow-up in this intolerant cohort. British Journal of Cancer (2008) The routine adjuvant endocrine standard of care for post menopausal women with oestrogen receptor positive breast cancer has shifted towards including an aromatase inhibitors (AI), following the evidence presented in five international trials demonstrating improved disease-free survival with AI containing regimens, compared to tamoxifen alone (Baum, 2001;Mouridsen et al, 2001; Coombes et al, 2004;Boccardo et al, 2005;Jakesz et al, 2005).It is, however, unlikely that all post menopausal women, particularly those with a good prognosis, where the relapse rate is low on either drug, have a clinically relevant advantage (Baum, 2001). Subgroups of patients have been identified who would benefit the most from AIs over tamoxifen, and particularly benefit from initial selection rather than sequencing a switch at 2 years. These include pathological prognostic factors which predict a higher and earlier relapse rate, such as histological grade, presence of vascular invasion, tumour size and number of positive axillary nodes (Mauriac et al, 2007); Biological molecular markers such a Cerb2, Ki67, CYP-2D6, level of oestrogen receptor positivity, progesterone receptor negativity (Ellis et al, 2001;Mauriac et al, 2007); Patient-related factors such as a history of thromboembolic disease, risk of uterine carcinoma or osteoporosis (Ellis et al, 2001;Lonning et al, 2005;Mauriac et al, 2007). The roles played by quality of life, tolerance and patient preference in the choice for either AI or tamoxifen; however, has not been formally established previously.Menopausal symptoms and overall quality of life have been extensively investigated within adjuvant breast cancer studies (Baum, 2001; Coombes et al, 2004; BIG 1-98 Collaborative Group, 2005;Boccardo et al, 2005;Jakesz et al, 2005), wi...