The cytokinesis-blocked micronucleus assay (CBMN) is a short-term mutagenesis test which offers an easier and less tedious alternative to metaphase chromosome analysis, with the advantage that exposure to both clastogens and aneugens may be detected. The CBMN assay has been used in evaluating the genotoxic consequences of exposures to environmental and occupational mutagens and carcinogens. Micronucleated cell rates (MN cell rates) were assessed in cytokinesis-blocked lymphocytes of 70 male and female cancer patients prior to any anticancer treatment. The study of interindividual variation factors showed that only age significantly affect MN cell rate, whereas sex, tobacco, alcohol, imaging techniques and tumour stage had no significant effect. The comparison of micronucleated cell rates in 198 healthy subjects and 70 cancer patients matched for age and sex showed a statistically significant difference. Spontaneous elevated MN cell rates of cancer patients refer to previous exposition of genotoxic or mutagenic environmental agents. Moreover, the MN cell rates in cancer patients most probably refers to various cellular lesions and genetic damages.