This study examines the longitudinal relationships between child smoking and secondhand smoke exposure (SHSe). Participants were 222 parent–child dyads. The parents smoked, had a child with (48%) or without asthma, and were enrolled in a smoking/health intervention. Parent-reported child SHSe was measured at baseline and 4, 6, and 12-month follow-ups; self-reported child smoking was assessed at these points and at 2-months. A parallel process growth model was used. Baseline child SHSe and smoking were correlated (r = 0.30). Changes in child SHSe and child smoking moved in tandem as evidenced by a correlation between the linear slopes of child smoking and SHSe (r = 0.32), and a correlation between the linear slope of child smoking and the quadratic slope of child SHSe (r = − 0.44). Results may inform interventions with the potential to reduce child SHSe and smoking among children at increased risk due to their exposure to parental smoking.</p