Background: Although vasomotor symptoms induced by aromatase inhibitors are frequently recognized, risk factors are ill-defined. To identify risk factors for vasomotor symptoms of Japanese breast cancer patients treated with adjuvant anastrozole, we conducted a prospective cohort study based on patient-reported outcomes (PROs). Patients and Methods: For this prospective cohort study (SAVS-JP, UMIN000002455), 391 postmenopausal Japanese estrogen receptor-positive breast cancer patients who were treated with adjuvant anastrozole were recruited from 28 centers. The PRO assessment was obtained from a self-reported questionnaire at baseline, 3, 6, 9 and 12 months between August 2009 and April 2012. Vasomotor symptoms, comprising hot flashes, night sweats, and cold sweats, were categorized into four grades (none, Grade 1: mild, Grade 2: moderate, Grade 3: severe). Pre-existing symptoms were only included if they worsened post-baseline. Results: Hot flashes, night sweats, and cold sweats at baseline were reported by 80 (20.5%), 59 (15.1%), and 32 (8.2%) patients respectively. New or worsening symptoms were experienced by 139 (38.4%), 106 (29.3%), and 104 (28.7%) patients. About 80% of newly occurring symptoms were grade 1, and less than 5% were grade 3. Overall vasomotor symptoms were reported by 201 (55.5%) out of 362 patients. Patients with vasomotor symptoms were significantly younger, had higher body mass index (BMI), shorter time period aftermenopause, and more menopausal disorders during menopause. Multivariate analysis showed that BMI (odds ratio: 1.09, 95% confidence interval: 1.02-1.16, P=0.009) and experience of menopausal disorders (2.11, 1.35-3.30, P=0.001) were significantly associated with vasomotor symptoms. Conclusions: We found that high BMI and experience of menopausal disorders at menopause were significantly associated with occurrence of vasomotor symptoms. These data are expected to prove useful for management of postmenopausal Japanese women treated with aromatase inhibitors. Clinical trial information: UMIN000002455.