Objective To examine the proportion of cases with proliferative endometrium on biopsies performed after positive home urine LH testing. Design Multicenter clinical trial of the usefulness of endometrial biopsy in the evaluation of infertility, with women from fertile and infertile couples randomly assigned to midluteal vs. late luteal phase endometrial sampling. Setting Twelve clinical sites of the National Institutes of Health/National Institute of Child Health and Human Development-sponsored Reproductive Medicine Network. Patient(s) All women in the study had regular menstrual cycles. Fertile volunteers who had delivered a live born infant within the past 2 years without medical intervention were recruited through advertisements at participating sites. Infertile women with regular cycles were recruited from the clinical practices of the sites' physicians. Intervention(s) Interview, informed consent, subject-interpreted home urine LH testing, and endometrial biopsy in either the midluteal or late luteal phase. Main outcome measure(s) Proportion of cases with proliferative endometrium on biopsy. Result(s) In both fertile and infertile women, more than 7% of endometrial biopsies performed 7–13 days after a positive home urine LH test revealed proliferative endometrium. Conclusion(s) Patient interpretation of home urine LH test kits not uncommonly results in false-positive tests. Women planning menstrual cycle testing or procedures related to ovulation may benefit from additional confirmatory testing.