A longitudinal randomized trial tested the self-determination theory (SDT) intervention and process model of health behavior change for tobacco cessation (N = 1006). Adult smokers were recruited for a study of smokers' health and were assigned to intensive treatment or community care. Participants were relatively poor and undereducated. Intervention patients perceived greater autonomy support and reported greater autonomous and competence motivations than did control patients. They also reported greater medication use and significantly greater abstinence. Structural equation modeling analyses confirmed the SDT process model in which perceived autonomy support led to increases in autonomous and competence motivations, which in turn led to greater cessation. The causal role of autonomy support in the internalization of autonomous motivation, perceived competence, and smoking cessation was supported.