Retrospective studies have suggested that blood pressure (BP) may be affected by a covert gender interaction between patient and observer, which is most marked for female patients and male observers. Such an effect could be one of the determinants of the "white coat' response. We have assessed this interaction in a prospective study in 56 hospital outpatients (31 males, 25 females) in whom supine and erect BP and heart rate were measured by two male and two female subjects, and by an automatic sphygmomanometer when no persons were present. Supine systolic BP (SBP) was related to age, and fell progressively over three measurements. Absolute BP was less in females but the fall on repeated measurement was significantly greater than in males. When absolute BP was analysed by patient and observer sex, no significant interaction was present. BP measured by automatic sphygmomanometry was less than the value obtained clinically. Absolute BP showed no significant interaction with observer and patient sex, but the difference between machine and age-adjusted clinical measurements for SBP was significantly greater in female patients when BP was measured by male observers. However, this effect dissipated on repeated measurement. These data suggest that a transient gender interaction can contribute to the white-coat effect, especially in female patients when BP is measured by a male observer.