Although smoking has been considered a risk factor in causing pepticulcers, no study has examined the effects of job stress on the relationship between peptic ulcers and smoking.To establish a link between gastric or duodenal ulcers over two years and a state of perceived job stress, a questionnaire, including questions on demographics, smoking, history of peptic ulcer and perceived job stress was conducted. Follow-up surveys were carried out every six months to accumulate the data for this analysis and the time span of this follow up study was two years. To examine the role of perceived job stress on the relationship between smoking and peptic ulcers, stratified analyses were performed.Some specific causes of perceived job stress such as "Too much competition, " "Schedule is too tight or pressed to work too hard" had high estimated relative risks: 2.13 with 95% confidence interval (CI) of 1.09 -4.16 and 2.50 with 95%Cl of 0.98 - 6.40, respectively. Stratified analyses suggested an effect-measure modification of perceived job stress in the relationship between peptic ulcers and smoking. Multiplicative and additive models suggest positive interaction between perceived job stress and smoking. These results suggest that specific perceived job stress is an effect modifier in the relationship between the history of the peptic ulcer and smoking. J Epidemiol, 1999 ; 9 : 320-326