['To evaluate and compare in vivo expression of T helper type 1 (Th1) cytokines in the peri-implantation endometrium of infertile patients between natural and stimulated cycles.', 'An observational study.', 'A tertiary assisted reproduction center.', 'Infertile patients.', 'Uterine flushings and endometrial biopsies were collected 7 days after LH surge in natural cycles or after hCG injection in stimulated cycles.', 'T helper type 1 cytokines were determined by immunolocalization and by ELISA.', 'Natural cycles were classified as group A (n = 17), whereas stimulated cycles with peak serum E(2) of <or=20,000 pmol/L (moderate responders) and with peak serum E(2) of >20,000 pmol/L (excessive responders) were classified as group B (n = 32) and group C (n = 32), respectively. Higher serum E(2) concentration was associated with increased expressions of interleukin (IL)-2 in the endometrium and uterine flushing. Group C demonstrated a statistically significantly higher IL-2 expression in endometrial biopsies by glandular and stromal immunostaining and by ELISA, compared with group A and group B. There was no difference in IL-2 concentration between group A and group B. Interferon-gamma protein concentration was comparable for the three groups.'] Is excessive ovarian response associated with increased expression of interleukin-2 in the periimplantation endometrium?