A histochemical method for the detection of steroid-binding sites in very small tissue specimens was evaluated. Competitive binding experiments proved the method to be sufficiently specific. When applied to endometrium and endometriotic tissue obtained simultaneously from the same patient, a distinct binding was found both in the epithelial structures and in the stroma. The binding pattern in endometriotic glands resembled that in the endometrium. The study seems thus to support the conclusion that endometriotic lesions contain specific binding sites for estrogen and progesterone and that their distribution seems to be similar to that found in the endometrium from the same patient.