 This is not a study that measures clinical outcome or safety of sildenafil. A single dose was administered and effect on diffusion capacity and 6 minute walk were assessed compared to baseline. While we would caution clinicians for rare exceptions to these findings, we believe our data exclude a significant confounding effect in interpretation of diffusion capacity in patients with parenchymal lung disease who are treated for pulmonary hypertension with oral sildenafil. Whether this conclusion holds true at higher doses of sildenafil and in patients with vasodilator response may offer further venues for research. 