This study examined whether an intervention designed to reduce secondhand smoke exposure (SHSe) among children being treated for cancer had effects in the specific setting of a motor vehicle. The parents or guardians (n = 71) of children being treated for cancer were randomized to either a behavioral secondhand smoke (SHS) reduction program or a standard care control group. Parental reports of SHSe were collected over the course of 12 months. Younger children were exposed at baseline more than their older counterparts. The greatest initial declines in car exposure were observed among children ≤5 years old in the intervention group compared with same-aged peers in the control group. After the 3-month time point, the control group showed greater reductions in car exposure in comparison with the intervention group. Interventions that teach parents strategies to manage their smoking while driving in their personal vehicles may produce even greater reductions in child exposure and should be developed. Based on the age-specific results reported here, future studies should account for effects of child age and use settingspecific measures of SHS.