Total-absorbed-iron requirements in adult and teen-age menstruating women were calculated from previously published data on menstrual blood losses; hemoglobin distribution in healthy, nonanemic women; basal iron losses; and growth requirements in teen-agers. Because present calculations included the effect of the variation of all indices, the new requirements for iron were increased to 2.84 mg/d in adult women and 3.21 mg/d in teen-agers (95th percentiles). Six independent estimations of whole-diet iron bioavailability were made to translate absorbed iron requirements into dietary requirements. In subjects with no iron stores, estimated dietary iron bioavailability amounted to 14% (Swedish diet), 16% (French diet), and 16.6% (US diet). When 15% was used as a single figure to represent the optimal, long-term bioavailability of iron in a general Western-type diet, the 95th percentiles of dietary iron requirements were 18.9 mg in adult menstruating women and 21.4 mg in menstruating teenagers.