The diagnosis of ruptured ulnar collateral ligament of the metacarpo-phalangeal joint of the thumb is made from characteristic findings and special radiograms. Without treatment, impairment of hand function will result. Only in a few cases will conservative treatment be successful; the reasons are explained. 26 cases of total rupture of the collateral ligament and their surgical treatment are reported. 16 fresh ligamentous injuries were treated by suturing directly (n = 2), by pull-out wire technique (n = 10), and by screw osteosynthesis (n = 4). The ligament was reconstructed with a free tendon graft in 8 cases of old ligamentous injuries, while the remaining 2 cases were repaired using a pull-out wire. After treatment, all metacarpo-phalangeal joints showed full stability and all patients without discomfort. Restriction of mobility results much more from secondary than from primary treatment. Therefore, we recommend primary surgical treatment of the ruptured ulnar collateral ligament of the metacarpo-phalangeal joint of the thumb.