Abstract Twenty-four hour urinary amino acid excretion has been studied in patients with β-thalassemia (four adult major, two adult intermedia, and three children major). β-Aminoisobutyric acid was found to be increased (14-fold on the average) in patients with β-thalassemia without evidence for increase in the excretion of most other amino acids. No correlation of β-aminoisobutyric acid excretion with the hematological status of the patient was found. Elevated taurine excretion (2-fold) was also noted in the adult patients with β-thalassemia. Greatly increased excretion of β-aminoisobutyric acid appears to correlate with a poor prognosis and may reflect generalized tissue catabolism rather than being a specific indicator of ineffective erythropoiesis.