In acute obstructive pyelonephritis, obstruction is the most dangerous factor. This danger is for the kidney where pyelocanalicular reflux is responsible for interstitial nephritis and pyonephrosis. A danger also for the life of the patient, since pyelo-venous and pyelo-lymphatic reflux are responsible for septicaemia. During a short period of two years, the authors have been able, thanks to cooperation of all physicians working in their teaching hospital group, to collect 89 cases of acute obstructive pyelonephritis, 29 of them with concomitant septicaemia. By virtue of their resolutely aggressive attitude, relief of urinary tract obstruction was obtained on average only 15 hours after hospitalisation of the patients. Fifty nine of these patients had lithiasis. Despite such rapid care, it should be emphasised that 11 patients died. This reflects the gravity of such retention of infected urine proximal to the upper excretory tract. This gravity is considerably multiplied by the onset of septicaemia. Among the 11 deaths, 9 were in the group of 29 patients with septicaemia, whilst there were only 2 deaths amongst the group of 60 patients with no clinical evidence of septicaemia. In the opinion of the authors, treatment should, whenever possible, include elimination of the obstruction and thorough urinary drainage. In the presence of any apparently isolated septicaemia, it is essential to seek a renal cause. The rapidity of the elimination of obstruction of the urinary tract is the essential step in treatment, antibiotics being only an adjuvant.