We report here a family of patients with coexistent hereditary spherocytosis and alpha-thalassaemia. The different clinical presentations of the family members were affected by the severity of the alpha-thalassaemia. The haemolytic effect of hereditary spherocytosis was modulated by the increased osmotic resistance of thalassaemia in the patients with both disorders. Coexistence of haemoglobin H disease and hereditary spherocytosis resulted in an asymptomatic state. In contrast, coexistence of alpha-thalassaemia trait and hereditary spherocytosis was still haemolytic. The different shapes of the osmotic curves in the family members correlated with the severity of their symptoms.