To study the role of the increase in follicle-stimulating hormone (FSH) during the early follicular phase of the menstrual cycle in folliculogenesis, eight normally cycling women were investigated in a spontaneous cycle (n = 8), a cycle treated with clomiphene citrate (n = 8), a cycle treated with FSH (n = 8) and a cycle treated with FSH plus pulsatile luteinizing-hormone-releasing hormone (LHRH) (n = 5). In the FSH and the FSH plus LHRH cycles, the leading follicle reached the size of 18 mm 2 days earlier on average than in the clomiphene-treated cycles. However, the rate of growth of the leading follicle was similar in the stimulated and the spontaneous cycles. At the same time, circulating levels of FSH differed greatly among the different groups of cycles. During the early follicular phase, serum FSH levels were significantly higher in the FSH plus LHRH and the FSH cycles than in the clomiphene and the spontaneous cycles. We conclude that the preovulatory stage of the dominant follicle is advanced by high FSH levels in the early follicular phase of the cycle. It is suggested that FSH controls follicle development in women at the recruitment--selection stage.