The spinal cord, brain, optic nerves and peripheral nerves may be affected by vitamin B12 (cobalamin) deficiency. Deficiency of vitamin B12 also causes megaloblastic anaemia, meaning that the red blood cells are usually larger than normal. In this paper we report a 16-year old girl who was referred to us for the evaluation of mild paraparesis and paresthesias marked by tingling "pins and needles" feelings and general weakness. The patient, her parents and sisters were on a strict vegan diet, which made us believe that vitamin B12 deficiency may be the possible cause of the neurologic clinical manifestations. The serum level of vitamin B12 was low, but there was no macrocytosis in the routine blood examination. The electrophoresis of haemoglobin was pathologic, there was 3.7% of HbA2 and 11.6% of HbF (heterozygous form of beta-thalassaemia). When megaloblastic anaemia occurs in combination with a condition that gives rise to microcytic anaemia, many megaloblastic features may be masked. Instead of being macrocytic, the anaemia could be normocytic or even microcytic. Vitamin B12 deficiency is a diagnosis that must not be overlooked. This case report turns the light on the fact that increased MCV is a hallmark in vitamin B12 deficiency, but it is not an obligatory sign.