The renal resistive index has been measured before and after hydration and administration of diuretics in persons with normal kidneys and in kidneys with small ureteral stone, either obstructing or nonobstructing, to assess induced flow changes and to identify features differentiating obstructing from nonobstructing stones. In normal kidneys the resistive index was normal (mean, 0.62+/-0.03); no changes in the resistive index occurred within 15 to 60 min after hydration alone, whereas the resistive index rose within 15 min after hydration plus administration of diuretics and then returned to initial values within 30 min. In both cases the resistive index decreased below basal values after 75 to 90 min. Similar changes were observed in kidneys with a nonobstructing ureteral stone. In kidneys with an obstructing ureteral stone the resistive index was higher than in normal subjects (mean, 0.73+/-0.02, P<0.01), increased further within 15 min after hydration and administration of diuretics (P<0.01), and remained higher than basal values during the following 90 min. In conclusion, different resistive index changes have been observed in response to hydration and diuretics in normal and obstructed kidneys. Duplex Doppler sonography and diuresis duplex Doppler sonography seem promising diagnostic tools to identify obstructing stones.