Idiopathic pulmonary fibrosis (IPF) is a form of idiopathic interstitial pneumonia characterized by temporally and spatially heterogeneous fibroblast proliferation and poor prognosis. No therapies have been shown in randomized clinical trials (RCT) to influence survival. Twenty-nine subjects were assigned randomly in a pilot study to a double-blind, placebo-controlled, RCT to test sildenafil in patients with IPF with forced vital capacity 40-90% and diffusing capacity 30-90% of predicted. During the 6-month experimental treatment period, patients underwent 6-min walk tests and estimation of dyspnea using the Borg scale at baseline (0 months), 3 months, and 6 months. Participants had moderate impairment of pulmonary function, and there were no significant differences between placebo (n = 15) and sildenafil (n = 14)-treated groups. Sildenafil did not significantly increase 6-min walk test distance (mean distance +/- SD after 6-month protocol: placebo 355 +/- 82 m, sildenafil 324 +/- 41 m; p = 0.256) nor did it lessen dyspnea after exercise (mean Borg score after 6-month protocol: placebo 3.4 +/- 1.6, sildenafil 4.1 +/- 2.3; p = 0.492). Adverse reactions were few and minor in nature. In this trial, sildenafil did not significantly increase 6-min walk test distance or decrease the Borg dyspnea index in patients with clinically typical IPF. This trial was registered at clinicaltrials.gov as NCT00359736.