Duplex ultrasound was performed in both kidneys of 156 normotensive pregnant women without suspected renal disease and 25 nonpregnant women of childbearing age to determine if resistivity indexes are different between pregnant and nonpregnant women and to assess whether the "physiologic" pelvicaliectasis of pregnancy causes elevated resistivity indexes. The mean intrarenal resistivity index for pregnant patients was 61.1, and that for nonpregnant women was 61.0. There were no statistically significant differences between the resistivity indexes of pregnant and nonpregnant women, regardless of whether the pregnant patients were considered as a composite group or subdivided by trimester. Twenty-nine right kidneys and four left kidneys in pregnant patients had moderate dilation of the collecting system attributed to physiologic pelvicaliectasis of pregnancy. No statistically significant differences were detected between the resistivity indexes of kidneys with and without pelvicaliectasis. Normal, uncomplicated pregnancy has no significant effect on resistivity indexes obtained in maternal kidneys. Physiologic pelvicaliectasis of pregnancy is likely to be partially a result of obstruction; this too, however, is not reflected by changes in resistivity indexes. An elevated resistivity index during pregnancy should not be attributed to the normal physiologic changes of pregnancy.