An examination of the applicability of the theory of planned behavior (TPB) to the intention to quit smoking across workplaces was conducted. Subjects were randomly selected from three workplaces in southern Taiwan. Those from a large public steel-manufacturing company were used for model building, and those from two private auto-parts-manufacturing companies served to cross-validate the model. Eligible subjects were divided into three study samples: a learning sample and two test samples. Three predictors--priority of quitting, past behavior (measured as previous quit attempt), and habit (measured as nicotine dependence)--were added to the TPB model. The results of this study show that TPB based on the learning sample fit well in another sample from the same workplace but poorly in other workplaces. When priority of quitting and past behavior were added to the TPB model, prediction to other workplaces significantly improved. Habit had no significant contribution to the intention to quit in the TPB model. Detailed discussions of the results are provided.