In the present work, we report two cases of monozygotic twins who developed prolonged apnea during electroconvulsive therapy (ECT) as a complication of anesthesia. In both cases, prolonged action of succinylcholine caused by a butyrylcholinesterase (BCHE) deficiency was confirmed by means of the dibucaine number test. In both cases, genetic analysis using the polymerase chain reaction revealed haplotype combined A and K variant mutations of the BCHE gene, both in the heterozygous form. These data show the potential risk of BCHE deficiency as a complication of anesthesia during ECT, and in particular underline the possible genetic contribution within a complex pathogenetic model.