In pursuit of a diagnostic modality better able to assess collecting system urine transport characteristics while operating within physiological ranges, a new set of guidelines for the pressure flow study was proposed. An infusion rate individualized for each patient was chosen based on a calculated estimate of the maximum physiological urine output, adjusted for patient size and age. The upper limit of normal renal pelvic pressure used was 14 cm. water. We evaluated 37 renal units with grade 3 or 4 hydronephrosis with the individualized pressure flow study. Patient age ranged from 0.2 to 12 years (median 1.1). Calculated individualized infusion rates ranged from 1.3 to 12.5 ml. per minute and resulting renal pelvic pressures ranged from 7 to greater than 40 cm. water. In each patient the corresponding renal pelvic pressure resulting from a fixed 10 ml. per minute infusion rate was uniformly equal to or higher than the corresponding individualized study pressures (p < 0.0001). Disagreement between the individualized and fixed rate pressure flow studies was highest in the younger patients. The correlation coefficient between diuretic nuclear renography half-times and individualized pressure flow results was 0.09, indicative of a random association between the 2 variables. By using individualized infusion rates based on the calculated estimate of the maximum physiological urine output, much of the falsely high pressures induced by nonphysiologically high fixed infusion rates in pediatric patients can be avoided.