A 15-month-old girl with severe hemolytic anemia and progressive respiratory failure is presented. She was well until the age of six months when she developed a pulmonary infection. During the next six months, she had frequent respiratory infections and her paleness became evident. At the age of 12 months, she was observed to have easy fatigability and muscle weakness, and she received her first blood transfusion. She was referred to our hospital at the age of 15 months. The physical examination revealed a malnourished girl with hypotonia, nystagmus, generalized muscle weakness and severe breathing difficulty requiring ventilatory support The hemoglobin (Hb) was 9.7 g/dl; hematocrit (Hct) 29%, mean corpuscular volume (MCV) 101 fl and reticulocyte count 15%. Peripheral blood smear revealed macrocytosis and stomatocytosis (30% of the red cells) and polychromasia. Sweat chloride test was 90 and 94 mEq/L on two separate occasions. The serum vitamin E level was 0.26 mg/dl (N: 0.44-0.68). She was found to be heterozygous for factor V Leiden mutation. Although malnutrition, low serum vitamin E and elevated sweat chloride test were suggestive of cystic fibrosis, this diagnosis failed to account for all the findings in the patient. A search for a red cell enzyme deficiency revealed that the red cell triosephosphate isomerase (TPI) activity was low. DNA analysis showed the 315 G-C (105 Glu-Asp) TPI mutation, thus confirming the diagnosis of TPI deficiency.