The main function of the enzyme butyrylcholinesterase is to hydrolyze exogenous esters such as those present in the neuromuscular blocking agent succinylcholine, which is frequently used in short surgical procedures. Inheritance of atypical butyrylcholinesterase or butyrylcholinesterase deficiency is considered an autosomal recessive characteristic and occurs worldwide in approximately one out of 3200 to 5000 people. Such deficiency may have serious consequences for patients anesthetized with that relaxant because sometimes it causes an extension of paralysis or postoperative apnea. We report the case of a 73-year-old woman submitted to subtotal thyroidectomy. After surgery, she did not spontaneously emerge from anesthesia, had signs of relaxation with poor inspiratory effort, and no recovery of neuromuscular response. After application of atropine, reversal was attempted with neostigmine, but it failed, and she was transferred to the ICU. Laboratory results revealed a reduced level of serum cholinesterase indicating a deficiency in butyrylcholinesterase consistent with her symptoms. Such situation has been described in medical literature.