Vitamin B12 is essential for proper neurological functioning, and its deficiency may cause a wide range of neuropsychiatric and hematological manifestations. We report a case of a previously healthy 32-year-old female who was admitted to our hospital with sudden onset of bilateral lower limb paraparesis and loss of sensation. The serum level of vitamin B12 was mildly decreased with high methylmalonic acid and homocysteine levels. However, her complete blood count showed no evidence of anemia or macrocytosis; instead, her mean corpuscular volume was low. Hemoglobin electrophoresis showed thalassemia trait, and that probably masked the megaloblastic features of vitamin B12 deficiency. She responded fully to vitamin B12 replacement therapy.