Problem/Objective: The objective of this study was to explore trends in cotinine levels in US worker groups to determine if exposure to workplace secondhand smoke is declining. Methods: Cotinine levels of workers in the NHANES III (1988-1991 and 1991-1994), 1999-2000, and 2001-2002 surveys who were not smokers and were not exposed to secondhand smoke in the home were analyzed with regression techniques to determine if there were significant trends in cotinine levels over time. The data were also analyzed by occupational/industrial and race/ethnicity-gender sub-groups. Results: Significant downward trends in cotinine levels were found in every group. Decreases from 1988-2002 ranged from 0.08-0.31 ng/ml (61-85% relative decrease). The greatest reductions were in the blue collar and service occupations, in the construction and manufacturing industrial sectors, in Non-Hispanic white male workers, and in black workers, irrespective of gender. Conclusions: All examined non-smoking worker groups with no reported home secondhand smoke exposures had declining cotinine levels over the years 1988-2002. The most dramatic reductions occurred in sub-groups which initially had the highest cotinine levels, providing evidence that the disparity in secondhand smoke exposure in the workforce is diminishing, presumably due to the increased adoption of clean indoor laws. Despite these successes, black workers, those employed in the construction sector, and blue collar workers continue to have the highest cotinine levels; additional research is needed to identify factors responsible for this continuing disparity. Further reductions in secondhand smoke exposure will necessitate widespread adoption of clean air laws which do not exempt any worker groups.