Inclusion Criteria1. between 4 and 9 days post-ischemic stroke;
2. admitted to inpatient rehabilitation;
3. cognitively able to provide consent or assent;
4. were living independently in the community prior to their stroke.
Exclusion Criteria1. hemorrhagic stroke or subarachnoid hemorrhage;
2. other neurological or psychiatric conditions deemed by the investigator to impair participation;
3. no previous stroke on same side of brain unless that stroke was cerebellar
4. moribound or not expected to live 6 months;
5. contraindications to taking sildenafil in pill or syrup form;
6. other medical conditions that would limit ability to participate in the study, specifically a) currently taking the P450 enzyme inhibitors (erythromycin, ketoconazole, itraconazole, saquinavir), b) currently taking the protease inhibitor ritonavir, c) currently taking nitrates or have a history of requiring nitrates treatment, d) have left ventricular outflow obstruction (i.e., aortic stenosis, idiopathic hypertrophic subaortic stenosis as confirmed by review of standard of care echo received before admission to rehabilitation), e) have severely impaired autonomic control of blood pressure, f) have previous history of symptomatic hypotension, g) have severely impaired hepatic function, h) is pregnant or become pregnant during the course of the study, i) Any medical condition which, in the opinion of the investigator, may compromise compliance with the objectives and/or procedures of the protocol.
7. scores > 50 on the upper extremity section of the Fugl-Meyer or no walking/gait impairment
For the MRI portion of the study only:
8. contraindication to MRI: ferrous metal in body, lead included body art, claustrophobia, pacemaker
9. allergic to or had previous reaction to gadolinium