Clinicians are often uncertain about how to manage elevated blood pressure (BP) when a patient reports that he/she has recently missed several doses of antihypertensive medications. While we know that better adherence can improve BP during several months, the magnitude of this relationship in the short term is poorly understood. The authors examined this issue using a group of patients who monitored adherence using a Medication Events Monitoring System (MEMS) cap and had BP measurements in the course of routine clinical practice. BP readings were compared following 7 days of excellent adherence (100%) or poor adherence (< 60%), omitting BP values following intermediate adherence. Using several different methods, BP following 7 days of excellent adherence was between 12/7 mm Hg and 15/8 mm Hg lower than after 7 days of poor adherence. Clinicians can use this effect size to calibrate their impressions of what the BP might have been with improved adherence.