The Resistance Index (RI) was studied in the main renal arteries in normal nonpregnant women and its change during normal pregnancy. One hundred and seventy color and pulse Doppler sonographic examinations were performed on 31 nonpregnant and 54 pregnant women. The range of gestation was 5-39 weeks, and patients divided into three groups; 5-12, 13-25 and 26-39 weeks of pregnancy, respectively. The RI was calculated in both main renal arteries. In nonpregnant women the left main renal artery RI mean = 0.587 +/- 0.021, and the right main renal artery RI mean = 0.584 +/- 0.028. In pregnant women the left main renal artery RI mean was 0.681 +/- 0.029, and the right main renal artery RI mean was 0.697 +/- 0.034. There was a significant difference between the main renal artery RI in pregnant and nonpregnant women (P less than 0.01). There was no significant difference between left and right main renal artery in pregnant and nonpregnant women (P less than 0.01). There was no correlation between gestational age and RI in the right main renal artery but a statistically significant correlation with gestational age in the left main renal artery (P less than 0.05).