The certainty of a strong genetic predisposition to malignant melanoma was first established over 35 years ago. Since it has been shown that constitutive chromosomal instability is significantly correlated with the familial occurrence of cancer, we have studied spontaneous micronucleus rates in fibroblast cultures from 44 melanoma patients, 44 healthy probands and 78 patients with bronchial carcinoma. Here we report a significantly (p = less than 0.0005) increased spontaneous chromosomal instability in patients with cutaneous malignant melanoma compared to healthy controls and other tumor patients (bronchial carcinoma).