Leptin and its receptor are involved in endocrine and paracrine regulation of metabolism, obesity and reproduction. Here, we describe the detection of the functional long isoform receptor of leptin in human endometrium. The leptin receptor protein was shown to be expressed in glandular and luminal epithelium and is periodically regulated throughout the menstrual cycle, demonstrating main expression in follicular and mid-luteal phase. In contrast, leptin receptor mRNA is detectable by reverse transcription-polymerase chain reaction (RT-PCR) as a constitutive component. Since RT-PCR analyses showed that leptin is not expressed in this tissue, the present study suggests that the human endometrium is a novel target for leptin. Therefore, we investigated 11 subfertile patients who underwent two biopsies in one menstrual cycle. The patients presented with a repetitive endometrial maturation defect, but showed adequate serum hormone concentrations and normal steroid hormone receptor expression and down-regulation in the endometrium. These patients were, however, deficient for expression of the functional leptin receptor. These analyses provide evidence that the lack of the leptin receptor in an ovulatory cycle may contribute to subfertility by a yet undefined 'endometrial factor'.