Recurrent gallstones are the limiting factor for every non surgical treatment. In order to determine the optimal prevention therapy, 30 patients after successful dissolution therapy of microcholelithiasis (MCL) with ursodeoxycholic acid (UDCA), were randomized by the double blind method to receive UDCA therapy. This was administered either as 150 mg (N = 15, group I) or 300 mg per day (N = 15, group II). There was a follow-up period of 12 months with clinical examination every month and upper abdominal sonography on days 180 and 360. Recurrent gallstones, after a 12-month period, was 6.7% (1/15) in group II versus 66% (10/15) in group I (P < 0.005). When recurrence was examined according to the number of gallstones, it reached 8% (1/12) in the solitary MCL vs 55.5% (10/18) in multiple MCL (P < 0.005). The recurrence was always symptomatic (biliary pain) and developed in 11 out of 30 pts, as MCL in 7 and as biliary sludge in the remainder 4. We conclude that a daily dose of 300 mg of UDCA was effective in reducing the recurrent gallstones in more than 90% of cases treated for 12 months. The significant risk factors associated with recurrence were the UDCA low dose and multiple stones. Biliary sludge represented 36% of the recurrence.