23,444 anaesthetic files were collected over a 12-month period from all the anaesthetic departments of the Regional Hospital in Nancy and three private clinics in town. The data were set out on a computer data-sheet aimed at defining the risk factors for histamine release. As a result, two groups are defined: a group of patients who released histamine (HL+; n = 212) and a group of patients who did not release histamine (HL-; n = 23,232). In increasing order of severity, the incidence of reactions were 1 in 143 for chest erythema, 1 in 630 for generalized erythema and Quincke's oedema, 1 in 230 for generalized erythema and hypotension, and 1 in 23,444 for fatal shock. The two groups were compared one with the other for each factor, using the chi-squared test or Fisher's test of exact probabilities. The statistically significant risk factors were: female patients, patients aged between 15 and 25 years, patients with an allergic history, a history of multiple anaesthesias, close one after the other, and exaggerated preanaesthetic anxiety. The drugs most often incriminated were: Althesin, propanidid, suxamethonium, alcuronium, gallamine and Haemaccel. A multifactorial analysis is being carried out to determine the part played by each factor in the release of histamine.