Blood pressure and heart rate were recorded at 15-min intervals for 24 hr in 60 untreated patients with uncomplicated mild essential hypertension using a new automatic noninvasive portable recorder. During the recording, the patients went about their normal daily routine, of which they kept a detailed record. The data were analyzed for five different recording situations: in the clinic, at work, at home, asleep, and average of the entire 24-hr period. Twenty-four hour readings were also compared with previously obtained casual readings. Clinic readings were correlated with the average 24-hr values, but for individual patients clinic pressures were relatively poor predictors of 24-hr pressures. Pressures recorded in the clinic were also greater than average 24-hr values. Similar degrees of correlation were found between clinic, home, work, and sleep pressures. Pressures recorded in the clinic were similar to pressures at work but higher than at home or asleep. In contrast, heart rate was similar in all conditions except during sleep, when it was lower. Previously measured casual pressures were also correlated with the clinic readings, with systolic values being similar, but diastolic values higher in the clinic during the 24-hr recording. For patients with clinic diastolic pressures in the range 90-104 mm Hg, 24-hr pressures varied from 75 to 100 mm Hg. We conclude that pressures measured casually in the clinic do not accurately reflect average 24-hr pressures and that ambulatory recording is helpful in the evaluation of mildly hypertensive patients.