ObjectiveTo investigate the effect of tadalafil (a long working phosphodiesterase type 5 inhibitor) on the endometrial thickness, biochemical pregnancy rates and clinical pregnancy rates in women in an in vitro fertilization treatment. This study investigates the use of vaginal and oral administration of tadalafil.Study designThis is a prospective double-blind placebo-controlled randomized controlled trial with 58 patients in an in vitro fertilization treatment with a short antagonist stimulation protocol. The study population is divided into three equal groups comparing oral and vaginal administration of tadalafil to a control group.ResultsNo significant difference in endometrial thickness and number of biochemical and clinical pregnancies was found between the three groups.ConclusionThis study could not show a significant benefit of administration of tadalafil. However, a trend towards more pregnancies in the group treated with oral tadalafil is seen, more research in specific subgroups is needed.