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Polycystic ovary syndrome, which is characterized by amenorrhea, is associated with insulin resistance. This association prompted investigators to examine whether menstrual-cycle length can be used to assess the risk for later development of type 2 diabetes mellitus (DM). Data were from the Nurses' Health Study II, a longitudinal study of 116,671 female nurses (aged 24 to 43 at baseline) who completed biennial questionnaires on lifestyle factors and disease between 1989 and 1997. The women recorded their usual menstrual-cycle length between ages 18 and 22, along with other demographic and health characteristics. Women who had DM at baseline and those who consistently used oral contraceptives between ages 18 and 22 were excluded. Women who s…