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Menstrual histories representing 38,194 woman-years were analyzed for seasonal periodicities using a simple additive model incorporating a secular trend. A recurrent sinusoidal pattern for cycle length was found which varied by 0.08 days throughout the year, decreasing during the warmer days of spring and summer, and increasing in the fall. Separate analyses of cycle variability and numbers of cycles indicated similar results but without as clear an effect. Although these variations were statistically different from random deviations occurring during the year, they were not large enough to indicate any physiological relationship