Injuries to the collateral ligaments of the proximal interphalangeal joints of the fingers are common and vary from simple strain to complete rupture, a condition which, if overlooked, frequently results in unilateral pain, swelling, and sharply localized tenderness and, when the ligament is completely ruptured, passive instability of the joint. This instability can be confirmed by proper stress roentgenograms. Review of our experience with injuries of one of the collateral ligaments of the proximal interphalangeal joints treated by non-operative means revealed that partial ligamentous ruptures responded satisfactorily while completely ruptured collateral ligaments frequently resulted in prolonged disability. Fourteen of eighteen fingers with completely ruptured collateral ligaments of the proximal interphalangeal joints were operated on with satisfactory results. Three patients refused operation; one was unsuitable for surgery.