Postmenopausal hot flashes are characterized by sweating and peripheral vasodilation and occur more frequently during increased heat loads. The circadian rhythm of core body temperature (T(C)) is well known and suggests that hot flashes will be most frequent when core temperature is highest. This hypothesis has not been tested previously. Ten symptomatic and six asymptomatic postmenopausal women were recruited from advertisements and screened. Each received 24-h ambulatory monitoring of sternal skin conductance levels to detect hot flashes, ambient temperature, skin temperature, and T(C). The last measure was recorded using an ingested radiotelemetry pill. Cosinor analysis demonstrated a circadian rhythm (P < 0.02) of hot flashes with a peak at about 1825 h. T(C) values of the symptomatic women were lower than those of the asymptomatic women (P <.05) from 0000-0400 h and at 1500 and 2200 h. The majority of hot flashes were preceded by elevations in T(C). Thus, elevated T(C) may serve as one trigger of menopausal hot flashes.