Behcet's disease (BD) is a chronic, multisystemic, inflammatory disorder characterized mainly by recurrent oral and genital aphthous ulcerations and uveitis. Our study aimed to determine the genetic damage in patients with BD. The micronucleus (MN) frequency was counted in peripheral lymphocytes and exfoliated cells of the patients with BD. MN analysis was performed in peripheral lymphocytes of 30 patients with BD and in 20 healthy controls by the cytokinesis-block method, and on uncultured cells of the oral cavity in 10 patients and 9 healthy controls. We found significantly higher MN rates in lymphocytes of the patients than the control subjects (P = 0.000). There were no significant differences between the patients with or without treatment (P = 0.860). The MN frequency in exfoliated cells of the patients was higher than in those of healthy controls (P = 0.013), and there was no significant difference between the exfoliated cells of the treated and untreated patients (P = 0.201). Our results indicate that genetic damage may play a secondary but important part in the aetiology of BD and that treatment with colchicine does not induce MN.