The length of the follicular phase of the menstrual cycle (defined as the time from the first day of menses until the day of urinary LH peak, inclusive) was examined in 30 healthy, premenopausal women. The women consumed defined, weight maintaining diets, with a ratio of polyunsaturated to saturated fatty acids (P/S ratio) of either 0.3 or 1.0. Both P/S groups consumed a high fat diet (40% energy from fat) for 4 menstrual cycles, followed by 4 menstrual cycles of a low fat diet (20% energy from fat). There was a significant increase (P less than 0.006) in the length of the follicular phase of the menstrual cycle during consumption of the low fat diet. Two thirds of the women showed increases in follicular phase length with an average increase of 1.9 days.