The naevoid basal-cell carcinoma syndrome (NBCCS) is an autosomal dominant disorder with multiple basal-cell carcinomas, an increased risk for other neoplasms, and various malformations. Chromosome instability has been implicated in the pathogenesis of this syndrome, but these reports are somewhat contradictory. We have investigated five patients, two with confirmed NBCCS and three suspected. No evidence for chromosome instability was found in lymphocytes at three sample times after stimulation using metaphase aberration analysis, sister-chromatid exchange (SCE) in second division cells, or micronuclei. A significant lengthening of the cell cycle was found for the two confirmed NBCCS patients, but not for the suspected cases.