Hot flashes are the most prominent side effect of tamoxifen, the most frequently prescribed antitumor agent in the world. Little detailed information is available to predict who will develop hot flashes on tamoxifen, to describe the natural history of these hot flashes, and/or to predict who will request therapy for such a side effect. This current trial was developed to address these items. Women who were about to begin adjuvant tamoxifen for locally treated breast cancer were approached for this trial. Before initiating tamoxifen, patients completed a short questionnaire designed to inquire about potential prognostic factors. Upon starting tamoxifen, women were asked to complete a hot flash diary daily for 3 months, and then daily for 1 week of each of the subsequent 9 months. Fifty patients, aged 51-83 years, provided data for this report. Approximately half of the women reported that they did not have any substantial hot flashes while the other half reported hot flashes of variable intensity. On average, these hot flashes gradually increased over 3 months and then plateaued. Baseline factors that appeared to predict for subsequent hot flash problems included a prior history of moderate to severe hot flashes with menopause and a history of prior estrogen therapy use. Overall, 16% of the women reported the desire for therapy for their hot flashes. Thirty-seven and one-half percent of the women (6/16) had a history of both prior estrogen use and moderate to severe hot flashes with menopause as compared to none (0/14) of the women without either of these factors. The data from this study can be utilized to better identify and educate women as to their probability of developing hot flashes after starting tamoxifen, to describe the average time frame for these hot flashes, and to predict the likelihood of whether resultant hot flashes will be substantial enough to have a woman request therapy for them.