Problem: Endometrial lymphocytes play a critical role in endometrial receptivity. This study aimed at evaluating the variations induced by chronic endometritis (CE) on endometrial lymphocyte subsets. We compared the results in infertile women diagnosed with CE with those in unexplained infertile women without any sign of CE. Method of study: Twenty-three women referring for unexplained infertility had hysteroscopy and endometrial biopsy in the follicular phase; in nine women, CE was diagnosed (group CE+), while in 14 it was not (group CE-). All patients in the late secretory phase of the subsequent cycle underwent endometrial biopsy. By flow cytometry, the percentage and phenotype of the endometrial lymphocyte subpopulations were analyzed. Results: The secretory endometrium of patients with CE displayed significantly lower percentage of CD56+ CD16- and of CD56bright CD16- cells (47.8% ± 18.6 and 30.1% ± 20.5 versus 79.5% ± 3.9 and 67.3% ± 8.1, respectively; P < 0.01) as compared with group CE(-), while the percentage of CD3 + cells was significantly higher (25% ±12.2 versus 10.5 ± 5; P < 0.01). Conclusion: Infertile women with CE showed an abnormal percentage of endometrial lymphocyte subsets compared with unexplained infertile women suggesting that different mechanisms underlie the adverse pregnancy outcome of the two groups of patients. © 2009 John Wiley & Sons A/S.