Summary—Patients with chronic retention of urine associated with dilated upper tracts have been investigated by means of combined lower tract urodynamics and upper tract renography. It has been shown that minor alterations in bladder volume lead to marked changes in isotope washout from the upper tracts. It is concluded that the practice of slow decompression of the bladder in these patients is clinically impractical, and attention should be directed to the management of the post‐obstructive diuretic state.Further observations have suggested that upper tract drainage in such patients may be dependent upon postural factors, and the practical implication of this finding is discus