Prostaglandin E2 (PGE2) was instilled into the bladder of 25 unselected patients suffering from post-operative urinary retention. Prior to this PGE2 application the patients were treated by a number of conservative methods without any success. 14 patients were unable to pass water spontaneously, 11 showed residual urines between 80 and 250 ml. As a rule 40 cc of an aqueous solution of PGE2 at body temperature was instilled into the catheterised bladder in the morning (14 times 0.75 mg and 11 times 1.5 mg) while the advice was given to avoid micturition for six hours. After this period and on the next morning micturition was allowed and the amount of residual was measured. When absolute urinary retention continued or the amount of residual urine exceeded 30 ml the instillation of PGE2 was repeated. Eight patients were able to empty the bladder at the first trial of micturition after PGE2 medication; on the next morning 3 additional patients were without residual urine. An obvious improvement of the initial situation was achieved in 12 additional cases (9 times spontaneous micturition for the first time and 3 times distinct diminution of residual urine). Only 2 test persons were treated without success. In 9 cases the instillation of PGE2 was repeated and thus 5 times complete emptying of the bladder was achieved as well as 4 times a marked reduction of residual urine. In 4 cases one to three days after successful PGE2 treatment retention of urine occurred again. No untoward side effects were observed.