Increased concentrations of mast cell tryptase are a highly sensitive indicator of anaphylactic reactions during anaesthesia. We obtained serum specimens from 350 patients after possible anaphylactic reactions during anaesthesia. Serum was collected from patients in our own institution (27), and transported by mail and courier from other hospitals in response to a request in the medical literature (323). Concentrations of mast cell tryptase were measured in 416 specimens. Intradermal testing was performed in 217 patients and radioimmunoassay for drug-specific antibodies with serum in 198 patients. Mast cell tryptase concentrations were increased in 158 patients, equivocal in 10 and not increased in 182. There was a significant difference in the incidence of positive intradermal tests, radioimmunoassay tests and evidence of an IgE-mediated reaction in patients whose mast cell tryptase concentrations were increased. Seven of 143 patients whose mast cell tryptase concentrations were not increased at appropriate sampling times had positive tests for IgE antibodies, and in 33 of 158 patients with increased mast cell tryptase concentrations no IgE antibodies were detected. We conclude that increased mast cell tryptase concentrations are a valuable indicator of an anaphylactic reaction during anaesthesia. Their presence favours an IgE-mediated cause but does not always distinguish between anaphylactoid and anaphylactoid reactions, and patients in whom mast cell tryptase concentrations are not increased still require skin testing.