OBJECTIVE: The incidence of vasomotor crises during the climacterium varies in women across different populations, and phytoestrogen consumption seems to play an important role in this problem. The aim of this study was to determine whether intervention with soy (dietary, extract, or concentrate), as compared with placebo, reduces the incidence of hot flashes in climacteric women. METHODS: Only published, randomized, placebo-controlled clinical trials were selected, with a 12-week duration of intervention, having selected postmenopausal women affected with hot flashes attributed to the climacterium (without cancer background). The intervention to be evaluated was soy, as "soy dietary supplement," "soy extract," or "isoflavone concentrate" (genistein or daidzein). The results were expressed as the number of hot flashes, average score of vasomotor symptoms, or average percent reduction in hot flashes within a time unit (day, week, or month). RESULTS: Nineteen studies were analyzed. The minimum heterogeneity was observed in the "isoflavone concentrate" group. In the "extract" and "dietary supplement" groups, heterogeneity reached an intermediate level, I = 42% and 59.73%, respectively. The overall result showed a standardized mean difference of-0.39 (95% CI,-0.53 to-0.25) in favor of soy as well as-0.45 (95% CI,-0.64 to-0.25),-0.51 (95% CI,-0.79 to-0.22), and-0.20 (95% CI,-0.46 to-0.06) for the "concentrate," "extract," and "dietary supplement" subgroups, respectively. CONCLUSIONS: Although the overall combined results and the results by subgroups (according to the type of supplement used) showed a significant tendency in favor of soy, it is still difficult to establish conclusive results given the high heterogeneity found in the studies. © 2010 by The North American Menopause Society.