Twenty-two patients with intermittent loin pain thought to be typical of primary pelvic hydronephrosis but with normal appearances at standard excretion urography were subjected to diuretic urography using Frusemide-induced diuresis. If this was normal, acute urography was performed when the patient had pain. Abnormal appearances were shown in only 15 patients. In 4, only the erect films revealed the hydronephrosis. Dilatation occurred in response to a diuretic load in 9 and the abnormality was shown at the time of an attack of pain in 4. The hydronephrosis was shown only during the attack of pain in 2, a diuretic urogram having been normal. In 2 patients the nephrographic signs of acute obstruction were shown to be due to occlusion of the pelvi-ureteric junction at the time of an attack of pain. The importance of sequential examination, the value of erect films and acute urography are stressed.