Butyrylcholinesterase (BChE) deficiency is a metabolic disorder characterised by prolonged apnoea after the use of certain anaesthetic drugs, including the muscle relaxants succinylcholine or mivacurium and other ester local anaesthetics. The duration of the prolonged apnoea varies significantly depending on the extent of the enzyme deficiency. The prevalence of BChE deficiency is highest in the Caucasian population with between 3.4 and 4% of the population displaying a partial enzyme deficiency leading to slightly prolonged apnoea (between 5 minutes and 1 hour) and 1 in 2500 individuals showing a prolongation of more than 1 hour. Individuals with undetectable levels of BChE activity display a severe prolongation lasting more than 8 hours. The prevalence of this severe form is estimated at 1 in 100 000 individuals. BChE deficiency is a multifactorial disorder. The hereditary condition is transmitted as an autosomal recessive trait and caused by mutations in the BChE gene. The BChE gene is located at the E1 locus on chromosome 3 (3q26.1-q26.2) and multiple atypical variants have been identified. However, BChE deficiency and sensitivity to anaesthetic drugs may also occur during pregnancy, in neonates or in association with other pathologies (chronic infections, malnutrition, liver disease, certain cancers etc.). Diagnosis can be made by analysis of enzyme activity in plasma samples, combined with dibucaine and fluoride inhibition tests. DNA analysis, although not carried out routinely, can be used to identify heterozygous carriers of atypical alleles. Affected individuals are asymptomatic unless exposed to neuromuscular blocking agents, however, prolonged respiratory paralysis following anaesthesia makes mechanical ventilation essential until the excess aesthetic agent is metabolised permitting normal neuromuscular function.