An increased chromosomal breakage rate (ICBR) was found in 27 of 28 patients with scleroderma (systemic sclerosis, SS) - 5 with the syndrome including calcinosis cutis, Raynaud phenomenon, esophagus hypomotility, sclerodactyly and telangiectasia (CREST), 4 incomplete CREST, 1 overlapping syndrome, 18 progressive systemic sclerosis (PSS). Not only the patients, but also about half of their first-degree relatives showed an increased chromosomal breakage rate (more than 5 breaks per 100 metaphases). This character segregated as a dominant marker in nine families of scleroderma patients. In the six informative of the nine families, the ICBR trait showed close linkage with the HLA region on chromosome 6 (total lod score 5.5 at theta = 0). In these families, ICBR was predominantly observed in linkage with HLA haplotype A1, Cw7, B8, C4AQ0B1, DR3 which is frequently observed in autoimmune diseases. The nature of the agent inducing chromosomal breakage in cultured lymphocytes of some, but not all family members of scleroderma patients remains to be clarified.