The three-dimensional (3-D) shape of erythrocytes is strongly associated with various diseases. However, conventional optical imaging approaches with Wright's staining only provide information on two-dimensional morphology. Here, we employed optical diffraction tomography (ODT), a label-free 3-D quantitative phase imaging technique, and observed uniquely shaped red blood cells (RBCs) in the peripheral blood of a patient diagnosed with myelodysplastic syndrome. Peripheral blood samples were collected when the patient visited our hospital for his two out-patient follow-ups in May 2018. The 3-D tomograms of randomly chosen RBCs were reconstructed using a commercial ODT setup. From the reconstructed 3-D RBCs, 37.5% and 32.8% of RBCs demonstrated cup-like shapes at the first and the second out-patient follow-up, respectively. Even though this is a single case report, the finding is novel and can be a potential dyserythropoietic feature found in peripheral blood.