A hereditary component is implicated in many different cancers, including hairy cell leukemia (HCL), and may involve an instability of the genome. We have previously documented recurrent clonal and non-clonal chromosomal abnormalities in hairy cells. To ascertain whether this instability of the genome is restricted to the malignant cells or if it might also include normal cells we performed cytogenetic investigations on skin fibroblasts and hairy cells from eight HCL patients and skin fibroblasts from eight referents. The frequency of chromosome abnormalities, regardless of clonality, was significantly increased in the fibroblasts from patients compared to referents. Also, five patients compared to one referent showed clonal abnormalities in their fibroblasts. Immunohistochemical investigations excluded the possibility that the fibroblast cultures were contaminated with hairy cells. Two patients had constitutional abnormalities, inv(5)(p13.1q13.3) and t(13;14), and one additional patient, possibly mosaic, showed the same abnormality, inv(9)(p21-22q22), in both fibroblasts (17/30) and blood (5/21) cells. Aberrations in patient fibroblasts also included sporadic inv(5), del(6)q, inv(19), and del(20)q, abnormalities previously shown to occur in hairy cells. A clonal expansion with trisomy 7 occurred in vitro as documented by fluorescence in situ hybridization (FISH). The only clonal abnormality occurring in a referent was -Y/-Y,+15 in an elderly male. In conclusion, a constitutional chromosomal instability may precede chromosome abnormalities and be of importance in the development of hairy cell leukemia.