Cockayne's syndrome is a form of cachectic dwarfism characterized by acute sun sensitivity and numerous other abnormalities of many organ systems. We studied fibroblasts from 9 Cockayne's syndrome patients to determine if their fibroblasts had abnormal post-ultraviolet light colony-forming ability or abnormal ultraviolet light-induced unscheduled DNA synthesis. The fibroblast strains from all the patients had markedly decreased post-ultraviolet light colony-forming ability in comparison with fibroblasts from control donors. Since this increased ultraviolet light sensitivity is propagable in vitro, it may be a manifestation of, or be closely associated with, the inherited genetic defect of this autosomal recessive disease. However, the patients' fibroblasts had normal rates of ultraviolet light-induced unscheduled DNA synthesis. Thus, unlike the UV sensitivity of DNA excision repair-deficient xeroderma pigmentosum strains, the UV sensitivity of Cockayne's syndrome strains is not related to abnormal DNA excision repair, at least to the extent that this repair process is reflected by rates of ultraviolet light-induced unscheduled DNA synthesis.