OBJECTIVE: To examine the secondhand smoke (SHS) exposure level in Chinese office buildings and to evaluate the effectiveness of a smoke-free policy in reducing SHS exposure. METHODS: Survey of smoking policies and measurement of SHS level in 14 office buildings from 10 provinces in China. RESULTS: Smoking in the building significantly elevated the SHS concentrations both in offices with at least one smoker and in offices with no smokers. In one building that recently adopted a smoke-free policy, the nicotine concentrations decreased significantly after the policy was enacted. Enactment of a smoking policy was effective in reducing SHS exposure in the buildings. CONCLUSIONS: Nonsmoking office workers in China were exposed to significant levels of SHS at work; both the central and local governments should realize the need to legislate against workplace smoking.