Chromosome aberration rates and sister chromatid exchange frequency were examined in the peripheral lymphocytes of 38 male workers who were engaged in organic glass production and exposed to methyl methacrylate (MMA) vapors at the concentrations of 0.9 ppm to 71.9 ppm. The results were compared with the findings in the concurrent nonexposed male subjects. Comparison of the exposed group with the nonexposed controls showed that there were no exposure-related changes in chromosome aberration rate. SCE frequency was higher in the exposed group than in the controls, but this was considered to be due to higher ages of the former group than that of the latter. In fact, selection of nonsmokers and further classification of the exposed nonsmokers into two groups of those with exposure below and above a median MMA concentration (ca. 4 ppm) failed to show any difference among the three nonsmoking groups in cytogenetic parameters, or any dose-dependency. The present results, although in a limited number of subjects, indicate that occupational methyl methacrylate exposure under the conditions studied is not associated with mutagenicity. This conclusion confirms the absence of mutagenicity of methyl methacrylate in humans, and is in general agreement with a majority of the results of studies on mutagenicity in vitro, animal carcinogenicity and occupational cancer epidemiology of methyl methacrylate.