The incidence of chromosome changes in men occupationally exposed to automobile fuels and exhaust gases was investigated. Blood samples were taken from the following four groups of 12 men: drivers of diesel-engine trucks, drivers of gasoline-engine trucks, automobile inspectors, and a reference group. The men in the groups were matched with respect to age, smoking habits, and length of time in their respective jobs. Chromosome preparations from lymphocytes were made and analyzed by standardized routine methods, and the frequencies of gaps, breaks, and sister chromatid exchanges were recorded. Statistical analysis of the results gave no evidence for a marked occupational effect, though there remains a grave suspicion that working with diesel motors may cause an increased level of chromosome changes. Smokers showed a significant or near significant increase in breaks, total aberrations, and sister chromatid exchanges over the nonsmokers in three of the groups. An analysis of the data revealed a weak positive correlation between the frequencies of chromosome breaks and sister chromatid exchanges in an individual.