5 patients with chronic myeloid leukaemia (CML) have been treated by BMT from identical twin donors. Syngeneity of the twins was established by conventional methods in the first 2 patients. These included the similarity of phenotype, dermatoglyphics and analysis of red cell isoenzymes and blood groups. 2 of the other patients had received multiple blood transfusions prior to referral for BMT thereby invalidating red cell analysis. However genetic identity was confirmed in these patients by the method of DNA 'finger-printing' which demonstrated identical restriction-fragment length polymorphisms. Conclusive proof of syngeneity in twins prior to BMT is important since it obviates the need for T-cell depletion and/or post-graft immunosuppression to prevent graft-versus-host disease (GVHD). All 5 patients were conditioned with high-dose chemoradiotherapy prior to BMT and all patients are alive and disease-free at a median follow-up of 12 months. In conclusion, we report a new, reliable method for determining syngeneity of twins which has bearing on the technical approach to BMT.