The aim of the present study was to define the different prevalence of hypertension when conventional office measurement and ambulatory monitoring are performed in a population of unselected workers. All the workers of a Florentine chemical company were invited to participate in the study. Enrolled subjects underwent blood pressure measurement using a conventional sphygmomanometer and ambulatory blood pressure monitoring. Of 191 workers, 145 agreed to participate in the study (76%). Six of the 145 were excluded from further analysis because they were undergoing antihypertensive therapy. Confidence limits for ambulatory monitoring were defined at 95% on normotensive workers. Thirty-five (25%) workers were found to be hypertensive according to World Health Organization parameters (diastolic pressure > 90 mmHg) but only 14 of the 35 had higher 24-h diastolic ambulatory blood pressure than the 95% confidence limits of controls.