Two sisters in whom a diagnosis of Fanconi's anemia was made at ages 12 and 18 subsequently developed acute nonlymphocytic leukemia (ANLL). A third sibling had previously died at age 11 of apparent sepsis. Both sisters had cytogenetic studies that showed increased chromosomal breakage and a 46,XX karyotype, but subsequently developed ANLL after, or coincident with, the emergence of monosomy 7. These observations suggest that, in addition to myelodysplastic syndromes and defective neutrophil chemotaxis, monosomy 7 may be associated with the emergence of leukemia in this disorder.