Three methods of assessment of upper urinary tract dilatation were compared prospectively. Forty-eight dilated renal units (35 patients) were studied, the majority being idiopathic pelvic hydronephrosis (69%). Dynamic computed tomography (CT) involves rapid sequence CT after injection of contrast medium. The maximum enhancement of a medullary region of interest (MMR) and its rate of enhancement (SMS) were derived from dynamic CT. Dynamic CT was compared with diuresis probe renography and measurement of parenchymal transit time (PTT) from DTPA imaging. The MMR (70.1 +/- 10.6 HU) and SMS (2.0 +/- 0.5 HU/s) in controls were significantly greater than in patients with upper tract dilatation (52.8 +/- 18.5 and 1.43 +/- 0.85 HU/s; both P less than 0.001). A significant association was observed between impairment of renal function in patients with upper tract dilatation and a low SMS and MMR (P less than 0.01). Statistically significant agreement was observed between the results of diuresis renography and PTT (P less than 0.025). However, no significant agreement was found when the results of dynamic CT were compared with either diuresis renography or with PTT. Dynamic CT as performed in the present study has no useful contribution to make in the diagnosis of patients with dilatation of the upper urinary tract.