The French--American--British co-operative study group (FAB) criteria were used to classify acute leukaemia in 194 adult patients. The dividing lines between FAB subgroups M1--M2 and M5A--M5B appeared to be less well defined. In an effort to improve subtyping we introduce the maturation index (MI). (formula: see text). Clearly distinct subgroups emerged after arrangement of cases according to MI values. Immature and rather mature cases in both myeloblastic and monoblastic leukaemia were divided at a MI of 4%. Assignment to M3, which is based essentially on morphological grounds, could be made on numerical grounds too, MI being at least 85% in all of our cases. A few cases showing M2 morphology and a positive alpha-naphthylacetate esterase (ANE) staining should probably be categorized as M4. Application of the original FAB criteria resulted in disagreement of 14% between three investigators, while with our additional criteria the discrepancy was only 2%.