OBJECTIVE To determine the clinical effectiveness of blood pressure (BP) measurement using conventional sphygmomanometry in the antenatal clinic and obstetric day unit compared with automated BP monitoring at home. METHODS The study population consisted of 109 nulliparous white women with BPs of at least 140 or 90 mmHg at the antenatal clinic after 20 weeks' gestation, who underwent obstetric day unit and 24-hour automated BP monitoring on the same day. Automated measurement was obtained every half hour for 24 hours using a commercially available device that had been previously validated in pregnancy. RESULTS At the traditional BP cutoff point (140/90 mmHg), the relative risk for subsequent development of adverse obstetric outcome was greatest for automated BP measurement: The relationships between outcome and automated diastolic BP were all statistically significant: proteinuria (P = .034), preterm delivery (P < .001), birth weight below the tenth percentile (P = .001), admission to the special care neonatal unit (P = .001), and cesarean delivery (P = .007). CONCLUSIONS Automated BP measurement appears to improve the identification of patients who are at high risk of poor obstetric outcome. Automated BP measurement is worthy of further evaluation as an antenatal screening and diagnostic test.