Hot flashes are experienced by about 52% of perimenopausal women. After breast cancer, this may increase to 70%. The use of hormone replacement therapy is not recommended in women who have had breast cancer; therefore, alternatives are required to help relieve hot flashes. This study was conducted to assess the efficacy of relaxation training in reducing the incidence of hot flashes in women with primary breast cancer. This was a randomized controlled trial of 150 women with primary breast cancer who experienced hot flashes. The intervention group received a single relaxation training session and was instructed to use practice tapes on a daily basis at home for one month; the control group received no intervention. Outcomes were incidence and severity of flashes using a diary and validated measures of anxiety and quality of life. The incidence and severity of hot flashes, as recorded by diaries, each significantly declined over one month (P<0.001 and P=0.01, respectively), compared with the control group. Distress caused by flashes also significantly declined in the treatment group over one month (P=0.01), compared with the control group. There were no significant differences between the treatment group and the control group at three months and no changes in anxiety or quality-of-life measures. Relaxation may be a useful component of a program of measures to relieve hot flashes in women with primary breast cancer.