Objective: Work stress contributes to risk of coronary heart disease and hypertension. This study tested the influence of job control on ambulatory blood pressure, and ratings of perceived stress and happiness in men and women systematically sampled by socio-economic status from the Whitehall II epidemiological cohort. Participants: A total of 227 men and women aged 47-59 years sampled from higher, intermediate and lower employment grades. Outcome measures: Ambulatory blood pressure and ratings of stress, perceived control and happiness. Methods: Participants completed standard measures of job demands and job control, and undertook ambulatory monitoring with measures of blood pressure and subjective state every 20 min from early in the working day until going to bed. Results: Systolic and diastolic blood pressure were greater in participants reporting low rather than high job control (means 125.7/81.5 versus 122.4/78.6 mmHg, P < 0.05), independently of gender, employment grade, body mass index, age, smoking status, and physical activity. Differences persisted into the evening after work. Job demands and job strain (high demand/low control) were not associated with blood pressure. Participants reporting low job control experienced stress more frequently over the working day than did those with high job control. Higher socio-economic status participants and women were more stressed by low job control than were men and people of lower socio-economic status. Conclusions: Job control plays an important role in modulating cardiovascular and affective responses over the working day, and these responses may contribute to increased cardiovascular disease risk. © 2004 Lippincott Williams & Wilkins.