Giulini S1, Carbone MM, La Marca A, Tirelli A, Radi D, Volpe A. Abstract PURPOSE: Nitric oxide (NO) intrauterine production has been shown to have an important role in the reproductive system in females. The objective of the present study was to evaluate NO concentration in endometrial washing throughout the menstrual cycle. METHODS: Observational study at Institute of Obstetrics and Gynecology, Mother-Infant Department, University Hospital. The study included 40 healthy fertile women, aged 21-40, with regular menstrual cycle, undergoing endometrial washing by hydrosonography for the assessment of NO concentration. RESULTS: Nitric oxide concentration in endometrial washing were low in early to mid proliferative phase (4.73 ± 1.57 mcM/L), but significantly higher (p < 0.05) in late proliferative phase (7.30 ± 3.37 mcM/L) early secretory phase (8.05 ± 1.97 mcM/L) and late secretory phase (8.69 ± 4.12 mcM/L) of menstrual cycle. CONCLUSIONS: Endometrial washing by hydrosonography is a simple, minimally invasive, and effective tool to use in the evaluation of cyclical NO intrauterine production. Nitric oxide concentrations increased during the late proliferative and secretory phase of menstrual cycle.