Despite many new therapeutic approaches to the treatment of hypertension and new diagnostic methods, the measurement of the most basic parameter, the blood pressure itself, has not changed over many years. The most commonly used method records the blood pressure on a single occasion and it is then assumed that this single reading is representative of the blood pressure of that patient at all times. This paper presents evidence, based on published studies, showing that the single "office" blood pressure reading gives little information about the blood pressure during daily activities. A study in 101 volunteers and a study in 38 patients with hypertension both showed that, not only is the "office" reading not the same as the average of many blood pressure readings made over 24 h using a Remler device, but also that the difference cannot be predicted. In some cases, the "office" pressure is higher and in some cases lower than the Remler blood pressure. This difference may explain the apparent blood pressure lowering effect of placebo. This has been frequently reported when the blood pressure is measured by the physician but ambulatory blood pressure readings have failed to show this effect. Finally, the inaccuracy of the "office" blood pressure may obscure smaller differences between two treatment groups in a clinical trial which could have been clearly demonstrated if the blood pressure had been recorded using an ambulatory method.