BACKGROUND Succinylcholine causes prolonged apneas in a proportion if subjects that have genetical defect of butyrylcholinesterase, due to the presence of unusual alleles in the locus BCHE. AIM To estimate allele frequencies of three variants of serum butyrylcholinesterase, BCHE*U, BCHE*A and BCHE*F in an urban population of Santiago, Chile. SUBJECTS AND METHODS Different phenotypes for the locus BCHE were determined in 300 blood samples coming from patients of a private clinical laboratory. The population was formed by an admixture of Amerindian and European (mostly Spanish) people. RESULTS The frequency of BCHE*A was similar to that expected for this population, but BCHE*F frequency was greater than predicted. Eight subjects had the genotype BCHE AK. CONCLUSIONS The higher frequency found for BCHE*F is probably due to the use of more precise detection techniques. Although the used method cannot distinguish BCHE UK from BCHE UU, the finding of individuals with BCHE AK, must lead to the suspicion that the frequency of the allele BCHE K is not negligible in Santiago.