The relative risk of nonunion in smokers versus nonsmokers after ankle arthrodesis was determined in a case control study. Twenty-two patients were matched to 22 controls by age, sex, surgeon, surgical technique, and preoperative diagnosis. Information on duration, amount, and past history of smoking was obtained for these 44 patients. Ten of the patients were actively smoking at the time of surgery compared with four of the controls. Six of the patients had no history of smoking and six had stopped smoking before surgery. Eleven of the controls had no history of cigarette smoking and 7 had stopped smoking before surgery. The relative risk of nonunion was increased 3.75 times for active smokers. When patients did not have any known risk factors for nonunion, the risk of nonunion for smokers was 16 times the risk of nonunion for nonsmokers.