Secondhand smoke exposure (SHSe) is a significant modifiable risk for respiratory health in children. Although SHSe is declining overall, it has increased for low-income and minority populations. Implementation of effective SHSe interventions within community organizations has the potential for significant public health impact. Objectives: To evaluate the effectiveness of motivational interviewing (MI) delivered in the context of a SHS education reduction initiative within Head Start to reduce preschool children's SHSe. Methods: A total of 350 children enrolled in Baltimore City Head Start whose caregivers reported a smoker living in the home were recruited. Caregivers were randomized to MI + education or education alone. Assessments were conducted at baseline, 3, 6, and 12 months. Measurements and Main Results: The primary outcome measure was household air nicotine levels measured by passive dosimeters. Secondary outcomes included child salivary cotinine, self-report of home smoking ban (HSB), and smoking status. Participants in the MI + education group had significantly lower air nicotine levels (0.29 vs. 0.40 mg), 17% increase in prevalence of caregiver-reported HSBs, and a 13% decrease in caregiver smokers compared with education-alone group (all P values 0.05). Although group differences in salivary cotinine were not significant, among all families who reported having an HSB, salivary cotinine and air nicotine levels declined in both groups (P 0.05). Conclusions: MI may be effective in community settings to reduce child SHSe. More research is needed to identify ways to tailor interventions to directly impact child SHSe and to engage more families to make behavioral change. (author abstract)