The presence and outcome effect of white coat hypertension in pregnancy was determined with 24-h ambulatory blood pressure (BP) monitoring. Sixty women presenting with high clinic BP (>/=140/90 mm Hg) in the second trimester were included. Patients were divided into two groups based on daytime ambulatory BP findings: <135/85 mm Hg, white coat hypertension (n = 37); >/=135/85 mm Hg, 'true' hypertension (n = 23). Complicated pregnancy outcome was defined as the presence of pre-eclampsia and/or intrauterine growth restriction. Groups were compared for pregnancy outcome and for background and delivery factors. The predictive value of ambulatory BP measurements for pregnancy outcome was determined. Pregnancy outcome was better in the white coat hypertension group than in the true hypertension group: pre-eclampsia-3 (8.1%) vs 13 (56.5%) (P = 0.0046); intrauterine growth restriction-5 (13.5%) vs 10 (43.4%) (P = 0. 0139); and preterm delivery-11 (29.7%) vs 15 (65.2%) (P = 0.015). Night-time ambulatory BP measurements were the best predictor of complicated pregnancy, followed by daytime and 24-h measurements. We conclude that second trimester ambulatory BP monitoring can be used to differentiate patients who have white coat hypertension, which is associated with a better pregnancy outcome than true hypertension.