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Objective Diurnal salivary cortisol patterns in healthy adults are well established but have not been studied in midlife women with hot flashes. We hypothesized that frequent hot flashes are associated with aberrant cortisol patterns similar to sleep-deficient individuals. Design Cross-sectional. Participants A total of 306 women, ages 40-62, randomized to a behavioural intervention for hot flashes. Measurements Baseline comparisons of cortisol geometric means (nmol/l) from four daily time points averaged over two consecutive days plus other calculated cortisol measures were made between groups defined by baseline: (i) mean daily hot flash frequency tertile (≤5·5, N = 103; >5·5-8·8, N = 103; >8·8, N = 100) and (ii) selected characteristics. Repeated-measures linear regression models of log-transformed cortisol evaluated group differences, adjusting for covariates. Results Women were 67% White and 24% African American, with 7·6 ( SD 3·9) hot flashes per day. Salivary cortisol geometric means (nmol/l) among all women were as follows: 75·0 ( SD 44·8) total, 8·6 ( SD 5·6) wake, 10·0 ( SD 7·5) wake +30 min, 3·7 ( SD 3·3) early afternoon and 1·6 ( SD 1·8) bedtime. Wake + 30-minute values showed an 18% median rise from wake values (interquartile range −24 to 96%), and means varied by hot flash frequency tertile, from lowest to highest: 11·4( SD 7·3), 10·3 ( SD 6·5) and 8·6 ( SD 7·8), respectively, P = 0·003. Beside the early afternoon value ( P = 0·02), cortisol values did not vary by hot flash frequency. Conclusion Taken together, these findings suggest that high frequency of moderate-to-severe hot flashes may be associated with subtle abnormalities in cortisol concentrations - a pattern consistent with chronic sleep disturbance. [ABSTRACT FROM AUTHOR]