An 83-year-old man underwent penetrating keratoplasty for pseudophakic bullous keratopathy in the left eye. Postoperatively, a thin transparent membrane in the anterior chamber was noted. The differential diagnosis included vitreous prolapse, retained capsule, fibrinous anterior chamber membrane, and retained Descemet's membrane in the anterior chamber, but the diagnosis was uncertain due to corneal edema and Descemet's membrane folds. Anterior segment optical coherence tomography was used to determine the diagnosis of retained host Descemet's membrane. This case report demonstrates that anterior segment optical coherence tomography is a useful and noninvasive instrument for diagnosing and following complications from penetrating keratoplasty.