To investigate the relationship between job-stress and blood pressure increase, 373 male blue collar workers without hypertension were followed for one year. 5 kinds of perceived job-stress were assessed by means of mailed questionnaires. Stepwise multiple regression analysis was conducted to examine significant determinants of blood pressure increases during follow-up. Job-stress due to complicated machine operation was found to be a significant predictor of diastolic blood pressure increase independent of other significant factors, i.e., systolic and diastolic blood pressure at the beginning of the follow-up, age, total serum cholesterol, alcohol consumption, type A behavior and family history of hypertension. Job-overload, physical discomfort, human relations and job-dissatisfaction, on the other hand, bore no significant relation to systolic and diastolic blood pressure increases. The results suggest that the use of production machines involving complicated operations and newly developed technology might be a risk factor for high diastolic blood pressure. © 1989, National Institute of Occupational Safety and Health. All rights reserved.