Hydroquinone (HQ) occurs in the environment as a result of manmade processes as well as in natural products from plants and animals. The compound has been reported to produce chromosomal effects in some in vivo and in vitro animal models. However, its potential to produce similar effects in human lymphocytes is less clear. To obtain more information on the clastogenic potential of HQ in human cells, its ability to induce structural chromosomal aberrations in human lymphocytes in vitro has been examined, both in the absence and presence of exogenous metabolic activation. Moreover, the effect of HQ pre-incubation on peroxide induced clastogenicity was studied, because HQ has putative chemopreventive activity as well. It was found that HQ was cytotoxic, but did not induce chromosomal aberrations in human lymphocytes cultured in vitro. Additionally, it was observed that pre-incubation of lymphocytes with HQ resulted in a concentration dependent reduction of the H2O2 induced chromosomal aberrations (P=0.069). However, this effect was present at 12 mM H2O2 only, because of high cytotoxicity at higher dosages. © 2003 Elsevier Ltd. All rights reserved.