The ambulatory blood pressure and heart rate of twelve healthy volunteers were monitored on two separate days in the participants' work environment. All subjects were studied in a double-blind crossover design while they consumed caffeinated or decaffeinated coffee. We examined them at rest, during a standardized mental arithmetic task, and for 6 h of ambulatory monitoring in their regular work setting (research facility and hospital). On days of drinking caffeinated coffee, both systolic and diastolic blood pressure were consistently elevated at all time points (P less than .0005). In response to the mental arithmetic task, the increase in systolic blood pressure over the resting value was particularly exaggerated on caffeinated coffee (P = .0003). For heart rate, there were no effects from caffeine. The study demonstrates that the blood pressure-elevating effects of caffeine, well-known from laboratory studies, are sufficiently robust to be evident in patients' uncontrolled work environments. It implies the importance of caffeine restriction for hypertensive patients. It also indicates the utility of ambulatory blood pressure techniques in studying the blood pressure effects of various nutritional factors taken ad lib in the real world.