OBJECTIVE: To determine the occurrence of the white-coat effect in patients already receiving antihypertensive therapy. METHODS: A review of data from several studies in which the prevalence of a white-coat effect was determined in different populations of patients receiving antihypertensive therapy. RESULTS: In an initial series of 71 treated hypertensive patients being studied in a tertiary care centre, we noted a white-coat effect (office minus ambulatory blood pressure >/= 20/10 mmHg) in 52 patients. This finding was confirmed in a larger series of similar patients with 106 of 152 having a white-coat effect. A white-coat effect was also more common in women (70 of 87) than in men (36 of 65). Similar findings were observed in untreated hypertensive patients in the community with 91 of 147 patients having a white-coat effect on the basis of routine office blood pressure readings taken by family physicians. CONCLUSION: The white-coat effect is common in patients being administered antihypertensive therapy and should be considered as part of their management in clinical practice.