In 17 of 50 cytogenetic studies (on chromosomal aberrations, micronucleus formation and sister chromatid exchange) in peripheral blood lymphocytes from a total of 667 workers in industries in which there is exposure to styrene, significant increases in the frequency of chromosomal damage have been reported when compared with unexposed controls. The positive or negative outcome of these studies is, however, unrelated to the extent of exposure to styrene. Furthermore, in the 17 investigations in which positive results were found, there is no convincing evidence of a positive dose-response relationship, in spite of the wide ranges of exposure and different methods of measurement. There are also serious discrepancies between findings on the chromosomal damaging effects of styrene in human lymphocytes in vitro and the types of damage reported in exposed workers. The findings are not consistent with the interpretation that styrene is responsible for the observed effects in workers. Several other chemicals identified in the environment of styrene workers have been reported to induce chromosomal damage in vitro and/or in vivo.