Two children presenting with sporadic unilateral retinoblastoma and exhibiting a high degree of chromosome breakage were noted to have unusual facies, microcephaly and abnormal skin pigmentation. In the first child the pattern of both spontaneous and mitomycin-C-induced chromosome breakage was characteristic of Fanconi's anaemia although the degree of breakage was extreme. She also exhibited a striking increase in X-ray-induced chromosomal damage in G0 lymphocytes as measured by dicentric formation and increase in chromatid-type aberrations. She had a number of typical clinical features, including cafe-au-lait patches and abnormalities involving the kidney; however, she demonstrated neither the hypoplasia of radius and thumb nor the typical aplastic phase of this disorder. At age 22 months the child became anaemic with trilineage myelodysplasia, which was rapidly followed by the development of acute myeloblastic leukaemia. The early onset (at age 4 months) of retinoblastoma may have been associated with the underlying genomic instability. The second child exhibited a pattern of chromosome breakage characteristic of Bloom's syndrome, in addition to a moderate increase in damage induced by mytomycin-C. She had the typical stunted growth and malar hypoplasia of Bloom's syndrome although she did not demonstrate the frequently described erythematous 'butterfly rash' Although patients with Fanconi's anaemia and Bloom's syndrome are recognised to be at an increased risk of cancer, retinoblastoma has not previously been described in patients with either condition. We suggest that underlying recessive chromosome breakage syndromes may be underdiagnosed in paediatric cancer patients, with important implications for prognosis and genetic counselling.