Dyskeratosis congenita (DC) is a rare inherited disorder characterized by bone marrow failure, dystrophic changes in the skin and mucous membranes, and a predisposition to malignancy. The DC locus has been mapped to Xq28. The primary defect responsible for this disease remains unknown. We have studied four patients with this disease, three from one family and one from another. In all four patients, primary skin fibroblast cultures were abnormal both in morphology (polygonal cell shape, ballooning, and dendritic-like projections) and in growth rate (doubling time about twice normal). Fibroblast survival studies using four clastogens (bleomycin, diepoxybutane, mitomycin-c, and 4-nitroquinoline-1-oxide) and gamma radiation showed no significant difference between DC and normal fibroblasts. Cytogenetic studies performed on peripheral blood lymphocytes showed no difference between DC and normal lymphocytes with or without prior incubation with clastogens. However, bone marrow metaphases from one of three patients and fibroblasts from two of four patients (who were the eldest of the 4) showed numerous unbalanced chromosomal rearrangements (dicentrics, tricentrics, and translocations) in the absence of any clastogenic agents. Cell-specific differences and a higher rate of chromosomal rearrangements in the older patients appear to correlate with the clinical evolution of the disease. These findings suggest that the DC defect predisposes DC cells to developing chromosomal rearrangements.