Children with cancer are at greater risk for the negative consequences of secondhand smoke exposure, making the identification of predictors of exposure critical. The current study investigated the impact of parents' psychosocial variables (perceived stress and vulnerability, self-efficacy), as well as health-related and demographic variables, on children's current exposure levels. Data were from 135 families whose children (M = 8.6 years old) lived with a smoker and were being treated for cancer. Self-efficacy was the consistent significant psychosocial predictor of exposure and the time since a child's diagnosis was indicative of lower exposure when limiting the sample to only smoking parents (n = 95). Both predictors of exposure have implications on motivation for behavioral change and may be suggestive of a teachable moment. Interventions may profit from tailoring programs to families based on these predictors of exposure, in particular for tobacco-based interventions for parents of medically compromised children, such as children with cancer.