OBJECTIVE: To examine the role of steroid hormone receptor compartmentalization in infertile women with "in-phase" or "out-of-phase" endometrium. DESIGN: Nonrandomized prospective clinical study. SETTING: A university clinic. PATIENTS: Twenty-nine infertile patients without evidence of endometriosis, tubal factor, male factor, galactorrhea, or hyperandrogenism were enrolled. Sixteen patients had in-phase endometrium and a P level > or = 10 ng/mL (conversion factor to SI unit, 3.18) (group A). Four patients had out-of-phase endometrium and a P level > 10 ng/mL (group B). Four patients had in-phase endometrium and a P level < 10 ng/mL (group C), and five patients had out-of-phase endometrium and a P level