Masseter muscle rigidity after suxamethonium, usually occurring in children induced with halothane,is associated with malignant hyperthermia. A case is reported in which masseter muscle rigidity occurred in an adult following vecuronium. From the limited data available, this and two similar reported cases, it appears that non-depolarizing muscle relaxants can, very rarely, cause masseter muscle rigidity in adults. This masseter muscle rigidity may complicate airway management, but is unlikely to progress to generalized rigidity and malignant hyperthermia.