It has been suggested that dilatation of the ureter seen in pregnancy is caused by progesterone. Twenty-four patients with ureteral calculi, therefore, were given an intramuscular injection of 250 mg hydroxyprogesterone in order to facilitate the discharge of the stones. Irrespective of the duration of symptoms there was a potential effect of treatment in up to 66% of the patients. In patients with symptoms of less than three weeks duration the stones were discharged within three weeks after the first symptom in 75% of the cases, which is a much higher incidence rate than that stated in the literature for spontaneous stone discharge (18-30%). In patients with longstanding symptoms 50% of the stones had passed, as compared to 39% reported in the literature. Especially concrements with a diameter larger than 4 mm had a better discharge rate. As the treatment is simple, cheap and without side effects, it seems worthwhile trying it in order to avoid surgery.