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Self-reported menstrual diaries were collected over a ten-week period from 36 women of a small tribal isolate in highland New Guinea known as the Gainj. All 36 subjects were healthy and were known from their serum hormone levels to be cycling; none was currently married, lactating or using any form of contraception. A survival analysis of the menstrual data indicates that the median cycle length of these women is 36 days, approximately 40% longer than the median estimated for a sample of U.S. women matched with the Gainj subjects for gynecological age. The difference between the U.S. and Gainj women is highly significant (logrank test statistic = 34.8, p < 0.0001). It thus appears that inter-population variation in human ovarian cycle length may be substantial.