We report a case of acute neck cellulitis and mediastinitis complicating a continuous interscalene brachial plexus block. A 61-yr-old man was scheduled for an elective arthroscopic right shoulder rotator cuff repair. A continuous interscalene block was done preoperatively and 20 mL of 0.5% bupivacaine and 20 mL of 2% mepivacaine were injected through the catheter. Postoperative analgesia was provided by a continuous infusion of bupivacaine, 0.25% at 5 mL/h for 39 h using a 240-mL elastomeric disposable pump. The day after surgery, the patient complained of neck pain. The analgesic block was not fully effective. He was discharged home. Three days later, the patient was readmitted with neck edema and erythema, fever and fatigue. Neck ultrasonography and computed tomographic scan revealed an abscess of the interscalene and sternocleidomastiod muscles and cellulitis, as well as acute mediastinitis. Two blood cultures and surgical samples were positive for Staphylococcus aureus. The infection was treated with surgery, the site was surgically debrided, and a 2-mo course of vancomycin, imipenem, and oxacilline. The technique of drawing local anesthetic from the bottle and filling the elastomeric pump was the most likely cause of infection. This case emphasizes the importance of strict aseptic conditions during puncture, catheter insertion, and management of the local anesthetic infusate.