Two episodes of hypotension caused by oral beraprost sodium administration following cardiac surgery are described. The first case was a 67-year-old female who underwent concomitant surgery for mitral valve replacement, tricuspid annuloplasty, and a radiofrequency maze procedure for atrial fibrillation. The second case was a 45-year-old female who underwent 4-vessel coronary artery bypass grafting associated with endarterectomy in the right coronary artery. Beraprost sodium was administered for the treatment of residual pulmonary hypertension in the first case, and was initiated as an antiplatelet agent following coronary endarterectomy in the second case. Hypotension occurred at approximately one hour after beraprost sodium administration in both cases. Careful observation to prevent this adverse effect is critical after the administration of beraprost sodium, especially in patients who have undergone cardiac surgery.