Abstract: To examine whether a family-based intervention targeting both smoking fathers and nonsmoking mothers in well-child health clinics is effective in increasing fathers' abstinence from cigarette smoking. This parallel 2-arm randomized controlled trial recruited a total of 1158 families with a daily-smoking father, a nonsmoking mother, and a child aged 0-18 months from the 22 maternal and child health centers in Hong Kong. The intervention group received the family-based intervention, including 6 nurse-led individual face-to-face and telephone counseling sessions within 1 month after recruitment and a voluntary face-to-face family counseling session (FCS). The control group received a leaflet, a self-help booklet, and brief quitting advice only. Father-reported 7-day and 6-month abstinence, smoking reduction, quit attempts, mother-reported help and support, and child salivary cotinine level were assessed at 12 months. Generalized estimating equation models were used to compare these outcomes between the 2 study groups. Compared with the control group, the intervention group reported a greater prevalence of 7-day (13.7% vs 8.0%; OR, 1.92; 95% CI, 1.16-3.17; P