The growth of the follicle was monitored in 61 subjects by ultrasonography and by the serial determinations of serum LH, FSH, estradiol (E2) and progesterone. All the subjects were judged to have normal luteal functions on the basis of their BBT patterns. The subjects were divided into two groups according to the length of their follicular phase: one with a follicular phase of 12--17 days (the control group, 39 cycles) and the other with a follicular phase of 18--26 days (the delayed ovulation group, 22 cycles). Ultrasonographically, the follicle grew slowly during the early follicular phase (slow growing phase), but began to grow more rapidly at 7 or 8 days before ovulation (rapid growing phase). In comparison with the control group, the slow growing phase was significantly prolonged in the delayed ovulation group. But in the length of the rapid growing phases and follicular growth rate, there were no significant differences between the two groups. In the serum levels of E2, LH, FSH and P, there were no significant differences between the two groups.