This is the first reported detection of serum IgE antibody to piperacillin-human serum albumin (HSA) conjugate in a patient presenting with anaphylaxis that developed after occupational exposure. A 24-yr-old nurse, who had worked at a University Hospital for 2 yr, experienced chest tightness, dizziness, generalized urticaria, abdominal pain, and diarrhea 10 min after administering a piperacillin injection. She had previously suffered from atopic dermatitis. A skin prick test for common inhalant allergens was entirely negative; in contrast, her serum total IgE was elevated (283 IU/mL). A high level of piperacillin-specific serum IgE was detected by ELISA using piperacillin-HSA conjugate. Significant inhibition upon addition of both free piperacillin and piperacillin-HSA conjugate was detected by inhibition ELISA. These data suggest that piperacillin exposure in the workplace can induce occupational anaphylaxis and urticaria mediated by an interaction of IgE with the hapten of piperacillin.