The study was conducted to estimate the frequency of white coat hypertension, ie, hypertension when in contact with the doctor only, in patients with newly diagnosed hypertension. We studied 159 consecutive patients (median age 47 years, 86 women) referred from primary health care to a hypertension clinic. For all patients, their general practitioner had decided to treat the hypertension pharmacologically, but the treatment had not yet begun. The blood pressure measurements reported from the referring doctors were 164.1 +/- 18.1/104.3 +/- 7.1 mm Hg (mean +/- SD). Measured at the hypertension clinic with random zero sphygmomanometer the pressures were 156.8 +/- 22.5/99.8 +/- 11.7 mm Hg. The indirectly measured day-time ambulatory blood pressures were 145.2 +/- 18.1/95.9 +/- 11.5 mm Hg. When a cut-off level of 90 mm Hg diastolic was applied, all patients were considered hypertensive by the referring doctors' measurements; the random zero office measurements regarded 18.3% of the patients as normotensive. The ambulatory measurements regarded a further 24.8% as normotensive (white coat hypertensive), whereas five patients considered normotensive by office measurements were hypertensive when evaluated by ambulatory monitoring (reverse white coat phenomenon). The difference between systolic office measurements and the daytime ambulatory blood pressures was significantly larger for female patients; age was positively correlated and weight negatively correlated to this difference. No significant difference was found for the diastolic measurements. It is concluded that white coat hypertension is present in approximately a quarter of the patients with newly diagnosed hypertension. By implementing ambulatory monitoring in clinical practice some of the overdiagnosing will possibly be avoided, but further studies on the subject are needed.