Thirty patients with the 8;21 translocation and three with closely related variants have been studied. Ages ranged from 3 to 64 years (mean 28.3). Thirty-one were entered into the MRC's 8th Acute Myeloid Leukaemia Trial. Twenty-nine (88%) achieved complete remission. Marrow smears from most patients showed granulocytic maturation (M2, FAB classification) with characteristic abnormalities, but at least six showed predominantly myeloblastic (M1) morphology. The blast cells were markedly heterogeneous with regard to size and nuclear cytoplasmic ratio. Typical staining patterns were observed in the blast cells using Sudan black B and diaminobenzidine peroxidase stains, and to a lesser extent with periodic acid-Schiff and chloroacetate esterase. Butyrate esterase was negative in all cases. Auer rods were present in the granulocyte precursors in 31 cases and in eosinophil precursors in two cases. In most cases the existence of the translocation was predicted from the cytological and cytochemical findings. Seven patients developed solid leukaemic deposits, principally in the mastoid cavities, orbital cavities or thoracic spine (extradural).