OBJECTIVE: There has been reduced active smoking, decreased societal acceptance for smoking indoors, and changing smoking policy since the mid-1980s. We quantified passive smoke exposure trends and their relationship with workplace policy. METHOD: We studied 2504 CARDIA participants (Blacks and Whites, 18-30 years old when recruited in 1985-86 from four US cities, reexamination 2, 5, 7, 10, and 15 years later) who never reported current smoking and attended examinations at 10 or 15 years. RESULTS.: In non-smokers with a college degree (n=1581), total passive smoke exposure declined from 16.3 h/week in 1985/86 to 2.3 h/week in 2000/01. Less education tended to be associated with more exposure at all timepoints, for example, in high school or less (n=292) 22.2 h/week in 1985/86 to 8.5 h/week in 2000/01. Those who experienced an increase in the restrictiveness of self-reported workplace smoking policy from 1995/96 to 2000/01 were exposed to almost 3 h per week less passive smoke than those whose workplace policies became less restrictive in this time period. CONCLUSIONS: The increasing presence of restrictive workplace policies seemed to be a component of the substantial decline in self-reported passive smoke exposure since 1985.