To provide an objective measure of the effects of on-call stress on the blood pressure (BP) of a group of pediatric residents, we used a SpaceLabs Ambulatory Blood Pressure Monitor (ABPM) to compare 37 pediatric residents' on- and off-call BPs. Residents wore the ABPM for 24 h (once on call and again off call) to assess systolic and diastolic BPs every 30 min during the day and hourly overnight. We found significantly higher MESOR (an acronym for midline estimating statistic of rhythm, which yields a mean value more representative of the true mean than an average of a series of measurements) BPs and BP loads (%BP readings > 135 mm Hg for systolic and/or 85 mm Hg diastolic) during the on-call period. Some residents became hypertensive on call, and the normal 24-h pattern of lower nighttime blood pressures was altered during this period. ABPM monitoring may prove useful in evaluating the effectiveness of interventions to reduce the stress of residency training.