This single center, prospective, open-label trial to assess the safety and efficacy of ranolazine in subjects with pulmonary hypertension associated with left ventricular diastolic dysfunction includes a screening period, 6 month treatment period and a follow up period. Eligible subjects who provide informed consent and who meet all inclusion/exclusion criteria will be enrolled in this study. All subjects will receive active study drug. There is no placebo group or use of control subjects. Treating will begin with 500mg twice daily and increased to 1000mg twice daily as tolerated after 2wks; the tolerated dose will be used for the remainder of the Treatment Period. Ranolazine will be limited to 500 mg BID in patients taking moderate inhibitors of CYP3A, including diltiazem, verapamil, aprepitant, erythromycin, fluconazole, grapefruit juice or grapefruit-containing products. During the Treatment Period subjects will be provided bottles of study medication to take home. Throughout the Treatment Period, subjects will return to the clinic for safety and/or efficacy assessments as well as resupply of Study Medication. Once the Treatment Period is complete, subjects will return to the clinic for a Follow-Up visit 14 days after the last dose of study medication, for a final safety assessment. Other therapies or possibly extension of the ranolazine treatment may be considered after completion of the Treatment Period at the discretion of the Investigator and will be recorded at the follow-up visit. Subjects who continue taking ranolazine (as part of there clinical care), will be asked to return for a 1 year follow up visit for safety and efficacy assessments.