A patient with features of Pearson's syndrome who presented with transfusion-dependent severe macrocytic anaemia, neutropenia, thrombocytopenia, and insulin-dependent diabetes mellitus in the neonatal period is described. His bone marrow was characterized by marked vacuolization of myeloid precursors and ringed sideroblasts. Autopsy examination revealed fibrosis and steatosis of the liver, reduction in the size and number of the islets, fibrosis and acinar atrophy of the pancreas, vacuolation of renal tubules, glomerulosclerosis, and "ragged red" fibres of skeletal muscles. Analysis of mitochondrial DNA (mtDNA) from the autopsied liver and skeletal muscle showed mtDNA heteroplasmy in both tissues, with one population of mtDNA deleted by 7374 bp. The deleted region was bridged by a single nucleotide, C, in normal mtDNA.