A prospective study was performed on 40 patients who were treated with a modified thumb spica cast (MTSC) for injuries involving the region of the metacarpophalangeal joint of the thumb. Twenty-seven of these injuries occurred during sports participation. The immobilization consisted of a fiberglass cast which incorporated the thumb in a balanced position and allowed full flexion and extension of the wrist. Twenty patients had injuries involving the ulnar collateral ligament (five surgical). Three patients had radial collateral ligament injuries (two surgical). Fourteen patients had fractures of the proximal phalanx, including five epiphyseal plate injuries. Two patients had dorsal dislocations and one patient had a metacarpal fracture. One patient with a nondisplaced distal metacarpal fracture was treated with simple immobilization. All patients had successful completion of their treatment within the usual time frame. There was no recurrence or increase in symptomatology during the treatment. There was no residual discomfort, loss of position or malunion of the fractures treated. The advantages of the MTSC include improved functional capabilities during immobilization, good patient acceptance, and no period of limited wrist motion or atrophy of forearm musculature. Our experience has been that the MTSC has been a reliable form of immobilization for treating injuries of the metacarpophalangeal joint of the thumb.