Luteal phase defect (LPD) accounts for a significant proportion of reproductive disorders, however its etiology is still debated. A prospective study was performed on 37 ovulatory women to determine whether LPD can occur in cycles characterized by completely normal folliculogenesis. Criteria for normal folliculogenesis included: a gradual rise of serum estradiol, a luteinizing hormone (LH) surge, the presence of a dominant follicle that disappeared, an increase of serum progesterone, and normal serum levels of prolactin, testosterone, dehydroepiandrosterone sulfate, follicle-stimulating hormone, and LH. Thirty of 37 women fulfilled the above mentioned strict criteria and underwent endometrial biopsy in the late luteal phase. Seven of 30 (23%) demonstrated a delay in endometrial development and all had normal hormonal and ultrasonographic parameters of folliculogenesis and ovulation. Women with delayed endometrial development demonstrated slightly longer follicular phases (17.0 +/- 1.1 versus 14.5 +/- 0.3 days). Perfectly normal follicular and periovulatory events may be followed by deficient luteal phases.