PURPOSE: Rupture of a collateral ligament of the metacarpophalangeal joint is rare except in the thumb. The injured digit became flexed and deviated toward ulna side by the hypothenar intrinsic musculature. Incomplete rupture of a collateral ligament of the metacarpophalangeal joint can be often managed by splinting the affected digit in flexion position, however, in the case of complete tears that distraction of the ends of the ruptured collateral ligament is too great to allow repositioning by splinting. Primary repair of the ruptured collateral ligament or reattachment to bone by a pull-out wire, or tendon graft technique appears to be adequate. METHODS: We report a case of instability of fifth metacarpophalangeal joint due to complete rupture of radial collateral ligament. This 18-year-old male presented pain in his right outstretched hand after trauma. The diagnosis was obtained by physical examination and simple radiography. Because of persistent instability after the initial conservative treatment, open reduction and repair surgical treatment was required. RESULTS: The fifth metacarpophalangeal joint became free of pain and stable under forced lateral deviation. Postoperative results showed good metacarpophalangeal joint function and stability during 8 months follow-up period. CONCLUSION: Because of the interposition of the sagittal band between the ruptured ends of radial collateral ligament such as Stener-like lesion of the thumb, surgical repair of metacarpophalangeal joint collateral ligament of the finger was justified in case of complete laxity in full flexion.