The vascularity of ten human flexor pollicis longus tendons was studied by injection and clearing techniques. Four separate vascular systems supply the tendon from the musculotendinous junction to the osseous insertion. The proximal two systems and the distal two systems are separated by an avascular zone over the metacarpophalangeal joint. While there are differences in the vascularity of the flexor pollicis longus tendon and the other digital flexor tendons, the intrinsic vascularization is similar. Differences in vascularity alone would not account for good recovery of function following repair of flexor pollicis longus.