Dual angle laser scattering cytometry (DALC) measures erythrocyte volume and haemoglobin concentration directly and provides spherocyte quantification. We have studied DALC red cell features in hereditary spherocytosis (HS) relating them to the clinical severity of the illness. A total of 140 controls and 80 non-splenectomized HS cases were studied, with mild (n = 32), moderate (n = 37) and severe (n = 11) disease. DALC measured mean corpuscular haemoglobin concentration, percentage of erythrocytes with haemoglobin concentration over 410 g/l (%Hyper), red cell volume and haemoglobin concentration distribution widths which were significantly different in HS cases when compared with controls (P < 0.001). Severe HS cases had more heterogeneous erythrocyte populations, both in volume and haemoglobin concentration (P < 0.001). Clinical severity of HS was best reflected by erythrocyte volume distribution width followed by %Hyper. DALC red cell features are both a diagnostic tool and an index to the severity of HS.