The brachial plexus sheath was examined in cadavers by using a combination of anatomic dissection, histologic preparations, and x-rays made after injection of x-ray contrast media, and in surgical patients by using computed tomography (CT) dye studies. The connective tissue forming the sheath was organized more densely proximally near its origin and became loosely organized distally as it ended by joining the medial intermuscular septum of the arm. The connective tissue forming the sheath extends inward, forming septa between components of the plexus. Thus, the sheath is a multicompartmented structure, formed by the thin connective tissue sheath surrounding the plexus and by the septa which extend inward from the sheath. A fascial compartment is created for each nerve, and this compartment serves to define the anatomic limits of that nerve. These compartments have potential clinical importance and implication in the techniques for brachial plexus block. They serve functionally to limit the circumferential spread of injected solutions of local anesthetics. These studies also indicate that injected anesthetic solutions spread easily in a longitudinal manner up and down the nerve and remain compartmentalized. The data presented here provide a rational explanation for the not uncommon occurrence of a profound block of rapid onset in one nerve, yet partial or absent block in other nerves, following any of the techniques of brachial plexus anesthesia.