1. With standard sphygmomanometric techniques used in conjunction with continuous intra-arterial monitoring the antihypertensive effect of placebo was examined in a group of patients participating in a controlled clinical trial.
2. Twelve of the 20 patients entered completed all stages of a randomized double-blind controlled trial of placebo and indoramin. Clinic blood pressure in patients receiving placebo showed reductions which were highly significant (P < 0.001). Simultaneous intra-arterial monitoring showed no reduction of the blood pressure over the 24 h studied.
3. The clinic pressures showed minimal reduction of blood pressure in response to indoramin therapy whereas intra-arterial pressures showed a significant fall (P < 0.01).
4. The response of the blood pressure to placebo appears to be an artifact of the indirect sphygmomanometric method and its use as a control in this study led to under-estimation of the efficacy of indoramin.