Occupational characteristics were used to study the role of job stress in the pathogenesis of hypertension. Ambulatory 24-h recordings of blood pressure were made for 161 men with borderline hypertension. From the occupational classification system scores for psychological demands, control, support, physical demands, and occupational hazards were obtained. The results indicated that the ratio between psychological demands and control (strain) was significantly associated with diastolic (but not systolic) blood pressure at night and during work. The association between job strain and diastolic blood pressure at night and during work was greatly strengthened when the subjects with occupations classified as physically demanding were excluded from the analysis. The conclusion was reached that a measure of job strain derived from the occupational classification is useful in predicting variations in diastolic blood pressure levels during sleep and work for men with borderline hypertension.