Electron microscopical and haematological investigation of peripheral blood has shown a higher percentage of leukaemia-like nuclear ultrastructural abnormalities in the leucocytes of 30 individuals with Down's syndrome (mean 6.3%) than in normal controls (mean less than 1%). Most of these aberrations consisted of nuclear membrane abnormalities. Red cell folate values were very low in the group with Down's syndrome. Although mean serum folate and vitamin B12 levels were normal in this group, these individuals displayed increasing macrocytosis and decreasing serum folate levels with age. The whole group with Down's syndrome showed an increased mean corpuscular volume (MCV). The percentage of ultrastructural abnormalities did not correlate with folate levels when they were analysed individually. The existence of nuclear membrane abnormalities and folate deficiency, both of which may be associated with increased chromosome breakage, may be partly responsible for the increased leukaemia risk in patients with Down's syndrome.