Objective To determine whether sildenafil citrate, a cyclic monophosphate-specific type 5 phosphodiesterase inhibitor, influences sperm motility or the acrosome reaction. Design Laboratory analysis of sperm motility after exposure to sildenafil citrate using computer-assisted semen analysis and acrosome reaction by fluorescein isothiocyanate-labeled peanut agglutinin staining. Setting An assisted reproductive technology (ART) unit. Patient(s) Fifty-seven male patients. Intervention(s) Sperm were divided into 90% (those with the best fertilizing potential used in assisted conception) and 45% (the poorer population) fractions by density centrifugation and incubated with sildenafil citrate (0.67 μM) at 37°C for up to 180 minutes. Main Outcome Measure(s) Both the number and velocity of progressively motile sperm were significantly increased by sildenafil citrate between 15 and 135 minutes. Furthermore, samples revealed that these effects were consistent in the 90% and 45% populations of sperm. In both populations, sildenafil also caused a significant increase in the proportion of acrosome-reacted sperm—22.1% compared with 11.8% in the control group of the good quality fraction and 16.6% compared with 9.4% in the control group of the poorer quality fraction. Conclusion(s) The use of sildenafil citrate may adversely affect male fertility.