Ceftriaxone, a third-generation cephalosporin with a long half-life, was administered intramuscularly to 30 adult women suffering from acute pyelonephritis. Standard bacteriologic methods were used to identify the pathogens and their susceptibilities before treatment and at intervals during and after treatment. Overall cure was achieved in 25 (85%) of the patients 6 weeks after completion of therapy. The results demonstrate that ceftriaxone is an effective drug when given once a day intramuscularly to out-patients with acute pyelonephritis.