We describe a pedigree with maternally inherited sideroblastic anaemia in which the red cells are dimorphic with a raised MCV. To our knowledge, this form of hereditary sideroblastic anaemia (HSA) has not been reported previously. 16 members of the family were investigated, revealing eight affected members. Two further family members were not tested but were presumed affected on the histories available. The proband, born in 1967, presented during pregnancy with a macrocytic anaemia (Hb 7.0 g/dl, MCV 106 fl) and a dimorphic red cell picture. Post partum, a bone marrow biopsy showed hypercellularity, mild dyserythropoiesis and ring sideroblasts. Cytogenetics were normal. Other causes of macrocytosis were excluded. Six other family members (three female, three male) have similar findings. There is no evidence of paternal transmission. An additional female relative who presented in 1992 with refractory anaemia with excess blasts in transformation and a dimorphic blood film, died from progression to AML. Affected members show a raised metal-free red cell protoporphyrin level suggestive of a defect at the level of Fe2+ incorporation into protoporphyrin. We propose that this form of HSA is due to a mitochondrial mutation. A search for deletions or point mutations in the mitochondrial DNA is currently underway.