Chromosomal breakage and sister-chromatid exchanges (SCE) were studied in peripheral lymphocytes of 9 patients with primary herpetic stomatitis (PHS), 12 patients with secondary herpetic stomatitis (SHS) and 12 controls. The incidence of chromosomal breakage was significantly higher in PHS patients (mean 23%, P. = 0.0002) and in SHS patients (mean 20.25%, P. = 0.0003) compared to the controls (mean 4.2%). The incidence of SCE per 46 chromosomes was significantly higher in SHS patients (mean 16.564) compared with (P. less than 0.001) the controls (mean 11.367) and compared with (P. = 0.006) the PHS patients (mean 12.131). It is concluded that both PHS and SHS patients exhibit structural chromosomal damage; SHS patients in addition exhibit more repaired chromosomal lesions.