To evaluate the effects of vaginally administered sildenafil on endometrial thickness and IVF outcome in a large cohort of infertile women with poor endometrial development. Retrospective cohort analysis. Private practice setting. A cohort of 105 infertile women aged <40 years, with normal ovarian reserve and at least two consecutive prior IVF failures attributed to inadequate endometrial development. Patients underwent IVF using a long GnRH-a protocol with the addition of sildenafil vaginal suppositories (25 mg, 4 times per day) for 3-10 days. Peak endometrial development, pregnancy, and implantation rates. Of 105 patients, 73 (70%; Group A), attained an endometrial thickness of >/=9 mm, whereas 32 (30%; Group B) did not. Implantation and ongoing pregnancy rates were significantly higher for Group A (29% and 45%) than for Group B (2% and 0). Of 11 women in Group B who had embryos transferred in that cycle, only one conception occurred, which resulted in a miscarriage. In Group B, 59% of women had a history of endometritis, compared with 44% in Group A. Vaginal administration of sildenafil enhanced endometrial development in 70% of patients studied. High implantation and ongoing pregnancy rates were achieved in a cohort with a poor prognosis for success. Previous endometritis may decrease the response to sildenafil.