To document more clearly the effect of selective medical treatment on the course of nephrolithiasis, we surveyed 103 consecutive patients being followed in our stone clinic. Of the patients who initially had existing stones within the kidneys 69 per cent experienced no symptoms while undergoing medical therapy, compared to 96 per cent of those who had no existing stones. In all subjects studied new stone formation was reduced in more than 95 per cent and no new stones were formed in more than 75 per cent. An operation for newly formed calculi was necessary in only 2 per cent of the patients on medical therapy, whereas 58 to 69 per cent of the patients required an operation for new stones before beginning medical treatment. We believe that selective medical treatment has an important role in the over-all management of nephrolithiasis. Appropriate medical therapy may decrease significantly the number of new stones formed and may obviate the need for a repeat stone operation. Therefore, medical treatment should be an important adjunct to percutaneous nephrostolithotomy and extracorporeal shock wave lithotripsy.