To determine if uterine artery Doppler velocimetry is useful in identifying chronic hypertensive pregnancies at risk for superimposed preeclampsia and adverse perinatal outcome. Resistance index (RI) was assessed by color velocimetry at the level of uterine arteries at 23-24 weeks of gestation in 42 women with chronic hypertension. The "lowest", the "highest" and the "average" values were compared to select the most predictive index for superimposed preeclampsia, intrauterine growth retardation (IUGR), birth weight lower than 2500 g and gestational age at delivery less than 36 weeks. Nine patients developed superimposed preeclampsia (21%) and 15 delivered before the 36th week of gestation (36%); 4 babies were IUGR (10%) and in 18 cases birth weights were below 2500 g (43%). Statistical analysis of Doppler findings showed that abnormal values of "lowest RI" were significantly correlated with adverse pregnancy outcome. Color Doppler analysis of uterine arteries is able to select chronic hypertensive pregnant women at risk of superimposed preeclampsia and poor perinatal outcome.