In order to test indirectly the hypothesis that Behçet's syndrome is caused by a virus, lymphocytes from eighty-six patients were evaluated for two parameters consistent with persistent virus infection: chromosomal abnormalities and decreased ability to herpes simplex virus type I (HSV) to grow in lymphocyte cultures stimulated by PHA. Whereas HSV grew in lymphocytes cultured from all normal donors, replication was impaired in lymphocytes from 37% of the patients with Behçet's syndrome. This figure is increased to 57% if patients receiving steroids or cytotoxic drugs were excluded. Lymphocytes were scored as chromosomally abnormal from sixteen of the thirty-eight patients examined, compared with only one of seventeen normal controls. There was damage to specific chromosomes in four patients. The frequency with which chromosomal abnormalities were detected was significantly related to failure to replicate HSV and inversely related to concomitant steroid treatment. The findings are consistent with a viral aetiology for Behçet's syndrome but other explanations are not excluded.