Peripheral erythrocytes of 5 patients with Behet's disease, not receiving systemic treatment, at the exacerbation stage, and those of 3 healthy controls were investigated with a transmission electron microscope. More than 100 cells were examined per subject. Cytolysosomes (autophagic vacuoles) were frequently observed in cells of the patients. The average frequency of appearance of cytolysosomes was 5.4 +/- 1.5% (mean +/- deviation) in erythrocytes of the 5 patients, and 0.5 +/- 0.4% in those of the 3 control subjects. The difference was statistically significant (p < 0.026). These results suggest that cytolysosomes of erythrocytes may reflect abnormal production or removal involved in the pathophysiology of Behçet's disease.