BACKGROUND Gender, age, smoking, race, and body mass index have been reported to determine the ambulatory white-coat effect (WCE) and white-coat hypertension (WCH). METHODS Baseline conventional, day-time ambulatory and self-measured home blood pressure measurements from the THOP trial were used to study the effect of gender, age, body mass index, smoking habits and treatment status on the white-coat syndrome as assessed by ambulatory monitoring or self-measurement. RESULTS The mean systolic/diastolic WCE was 9.1/6.7 mmHg if based on ambulatory blood pressure and 12.2/8.7 mmHg if based on self-measured blood pressure. The ambulatory WCE was significantly higher in women, in older subjects (65+), in obese subjects, in non-smokers and in patients on antihypertensive drug treatment. The self-measured WCE was significantly higher in women and in non-smokers. Ambulatory WCH was present in 6.6% of the untreated patients and 14.2% had self-measured WCH. The proportion of ambulatory WCH was significantly higher in obese subjects; the proportion of self-measured WCH did not differ by gender, age, body mass index, or smoking habits. CONCLUSIONS The ambulatory white-coat syndrome was determined by gender, age, body mass index, smoking habits, and treatment status. The self-measured white-coat syndrome was greater than the ambulatory white-coat syndrome but depended less on the determinants under study.