OBJECTIVES: To determine the magnitude of the placebo effect in studies examining the impact of hormone therapy (HT) on the frequency of hot flashes among menopausal women. We also examined the influence of data type on point estimates. METHODS: We searched computerized databases for English language articles from 1966 to April, 2004. Two reviewers evaluated 5840 published titles, identified placebo-controlled observations that met explicit inclusion criteria, and extracted data pertaining to study characteristics, interventions used, and outcomes measured. Studies evaluating the effects of HT in cancer patients were excluded. The magnitude of the placebo effect on frequency of hot flashes was calculated using a random-effects model. The effects of outcome type (binary and continuous) were assessed separately. RESULTS: Based on our criteria, a total of 25 studies were included in the meta-analysis. When the effects of data type were assessed, studies with continuous outcomes (N=18) reported a larger placebo effect than studies using binary outcomes (N=7). The difference in standardized mean difference (SMD)=2.52 (CI: 2.17-2.8). A larger placebo effect was observed in weekly hot flashes. The difference between weekly and daily SMD=0.5 (CI: 0.15-0.85). CONCLUSIONS: Placebos are used to control for natural remission and provide a standard for comparison to active treatment. However, it is difficult to distinguish a true placebo effect from reporting bias when studying hot flashes; patients tend to please investigators by reporting positive changes when no improvement took place. Overall, we found a greater placebo effect in studies with continuous outcomes possibly due to the systematic differences in the self-recording of symptoms or the natural decline of symptoms. These findings may apply to other patient reported outcomes in other conditions.