The present study was undertaken to evaluate objective methods of monitoring postmenopausal hot flashes. Continuous recordings of finger and core temperature and sternal skin conductance were carried out in 8 postmenopausal and 4 premenopausal women. Four postmenopausal patients were studied a second time following 30 days of daily oral administration of ethinyl estradiol 0.05 mg. One hundred twenty flashes were subjectively noted and recorded during 104 hours of study in the postmenopausal women. Eighty-two, 98, and 81% of subjective flashes were associated with changes in finger temperature, skin conductance, and core temperature, respectively. The rate of occurrence and magnitude of changes of physiologic markers were significantly greater (P less than .01) in postmenopausal than premenopausal women. Estrogen replacement therapy not only eliminated the subjective sensations but also significantly reduced (P less than .02) the frequency and magnitude of the changes in physiologic function measured by finger temperature. Measurement of skin conductance changes was the single most sensitive and specific indicator of hot flashes. The simultaneous change of both skin conductance and finger temperature, although less sensitive, was a very specific indicator of a hot flash. These results support the concept that the measurement of physiologic changes can be used to assess objectively the occurrence of this symptom complex.