A simple metabolic investigation was carried out in 54 patients presenting with their first attack of renal stones. In three cases a probable cause for stone formation was found. During the following 8 years 53% of the patients developed recurrences, the risk of which was highest during the first years after the initial attack. Recurrences were more common in males than in females. As a group, patients who were to have recurrences had higher urinary calcium levels but there was a considerable overlap between the two groups. This prospective study confirms the opinion from retrospective evaluation of the natural history of renal stones that recurrences are common. However, they cannot be predicted from standard laboratory findings in individual patients.