Patients with acute pyelonephritis may be extremely ill and have a complicating septicaemia. This may necessitate the parenteral administration of an appropriate antibacterial agent. This paper reports the use of intravenous co-trimoxazole in two young women with severe acute pyelonephritis. 10 ml of co-trimoxazole (800 mg sulphamethoxazole and 160 mg trimethoprim) was added to 500 ml of 5% dextrose in water and infused over a 90-min period. This treatment was given every 12 h for five doses, before switching to oral co-trimoxazole. The treatment was clinically and bacteriologically successful and no side effects were noted.