BACKGROUND Hereditary spherocytosis encompasses a heterogenous group of inherited disorders due to alteration of r.b.c. surface/volume ratio. Spectrin deficiency is the most common observed defect. We analyzed a case of HS associated with band 3 deficiency without spectrin reduction. METHODS In the study of a family originating from southern Italy, we show that a 20% deficiency of band 3 with normal spectrin content may be responsible for dominantly inherited hereditary spherocytosis (HS). The proband is a 12 years old girl consulting for jaundice, chronic anaemia and splenomegaly. Her mother had a similar haematologic phenotype. RESULTS Electrophoretic analysis of erythrocyte membrane proteins showed a deficiency in band 3 protein. Band 3 protein chymotryptic fragments, deglycosylated band 3, and its isolated cytoplasmic domain, all displayed normal electrophoretic migrations. Furthermore, the tryptic peptides profile of the cytoplasmic domain of the protein did not demonstrate any abnormality, nor did the amino acid composition of the peptides. Analysis of the membrane proteins during erythrocyte ageing, evaluated in density-fractionated red cells, showed that band 3 content was normal in the lighter fraction, whereas in the denser fraction band 3 deficiency was more pronounced than in membranes from non fractionated red blood cells. CONCLUSIONS This case describes HS due to anion exchange transporter deficiency. Our results on fractioned red cells support the hypothesis that the defect was probably due to a band 3 protein loss during cell ageing and not to a primitive quantitative defect.