Prerequisite to the use of ambulatory blood pressure monitors in epidemiologic research is demonstration of a satisfactory level of accuracy and precision. Previous evaluations of these devices raise a number of methodological concerns which complicate interpretation of their findings. Also, important issues regarding the precision of blood pressure measurements and the identification of factors associated with measurement inaccuracies remain unaddressed. To assess the accuracy and precision of the Accutracker ambulatory blood pressure monitor, we obtained five serial estimates of resting blood pressure on 120 ambulatory subjects in The Johns Hopkins Medical Institutions in 1987. Two Accutrackers and two manual observers independently recorded blood pressure with the order determined at random. The fourth observer in each sequence obtained a replicate measurement. For both systolic and diastolic blood pressures, the mean difference between Accutracker and manual measurements was less than 3 mmHg for each pairwise comparison. Scatter plots and regression analyses demonstrated that both Accutrackers tended to underestimate high systolic and diastolic blood pressures. In bivariate and multivariate analyses, increased age was significantly associated with underestimation of systolic blood pressures by both Accutrackers. With respect to the precision of blood pressure measurement, no significant differences were present among the four observers. In summary, our data suggest that the Accutracker has satisfactory accuracy and precision, but that accuracy is not uniform across patient subgroups.