The evaluation and treatment of injuries of the ulnar collateral ligament of the metacarpophalangeal joint of the thumb remain controversial. In a retrospective study that was done to assess our ability to determine whether displacement of the ligament (a Stener lesion) was present, we reviewed our accumulated experience with patients who had an injury of this ligament who were treated surgically between 1972 and 1984. Since our method of evaluation changed in 1977, we compared the preoperative and operative diagnoses in the twenty patients who were treated surgically from 1972 through 1976 with those in the twenty patients who were so treated from 1977 through 1984. Considering all forty patients who were treated operatively, sixteen (40 per cent) had a typical Stener lesion, and in two others (5 per cent) the ulnar collateral ligament was rolled up on itself and lying beneath the adductor aponeurosis. From 1972 through 1976, stability was tested with the metacarpophalangeal joint in complete extension or in varying amounts of flexion. Of the twenty thumbs that were evaluated by this technique and were treated surgically, 20 per cent had a Stener lesion. From 1977 through 1984, stability was tested with the joint in full flexion because of the findings in anatomical studies that were completed in 1977; the incidence of a Stener lesion in the twenty thumbs that were treated by repair or reattachment of the ligament during this time was 70 per cent.(ABSTRACT TRUNCATED AT 250 WORDS)