Ten volunteers (males and females) each had their hair dyed 13 times at intervals of 3–6 wk. Each volunteer used a single commercial preparation throughout the study. The preparations used contained a mixture of aminotoluenes, aminophenols and hydroxybenzenes and, in some cases, naphthol, as the active ingredients. Lymphocytes of the hair-dyed volunteers and of ten controls matched for age and sex were scored for chromosomal aberrations. The incidence of aberrations did not differ significantly between the controls and the hair-dyed volunteers at any of the nine sampling times (before the first exposure, after the first (sham) dyeing and then after each of the next three and the last four dyeing procedures). An increase in the aberration rate with time was observed both in the controls and in the hair-dyed subjects. The reason for this increase could not be determined. No clastogenic effect of repeated hair dyeing was established in this study.