The Gorlin syndrome, or naevoid basal-cell carcinoma syndrome (NBCS) is an autosomal dominant disease. It has been suspected for long that this cancer prone disease (multiple basal-cell carcinomas; other malignant or benign proliferations) is a chromosome instability syndrome. We previously reported a lengthening in the cell cycle of lymphocytes from two patients with NBCS. With a larger sample (n = 7), we confirm this disease to be a chromosome instability syndrome, although clearly, expression of this characteristic can vary between patients: (1) spontaneous chromatid breaks occurred more often in a subset of the patients; (2) spontaneous micronuclei were found more frequently in NBCS than in the controls; (3) we confirm the cell cycle to be affected in this disease. As these results were obtained on lymphocytes--a cell lineage not affected in NBCS manifestations--the chromosome instability we found would appear to be part of the general condition of this syndrome.