Inhibition of phosphodiesterase 5 is an attractive candidate mechanism for blood pressure (BP) lowering. In this study, a novel long-acting phosphodiesterase 5 inhibitor, PF-00489791, was evaluated in 133 patients with mild to moderate hypertension, randomized into 1 of 4 groups: placebo, 4 mg, 10 mg, and 20 mg titrated after 14 days of dosing to 40 mg. Study medication was administered once daily for 28 days. Ambulatory BP monitoring was used. There was a statistically significant decrease (compared with placebo) in mean daytime systolic BP on day 28 at the 10 and 20/40 mg doses (by approximately 5 and approximately 7 mm Hg, respectively). Changes in mean daytime diastolic BP corresponded with those in systolic BP. The magnitude of BP lowering was greater on day 1 than on days 14 and 28, but the response was sustained between days 14 and 28. PF-00489791 also exerted BP lowering effects on mean 24-hour ambulatory BP. There was a dose-related increase in plasma cGMP concentration (statistically significant at the 20/40 mg dose). There was an increased incidence of headaches at the 10 and 20/40 mg doses (22% and 21%, respectively, compared with 12% with placebo) and an increased incidence of dyspepsia/gastroesophageal reflux disease and musculoskeletal adverse events at the 20/40 mg dose. In conclusion, PF-00489791 causes a clinically meaningful and sustained BP lowering in patients with hypertension. It is generally safe and well tolerated at the clinically efficacious doses.