Fourteen individuals with low serum cholinesterase activity were examined by genetic analysis. All of these were considered to be due to silent gene because their dibucaine number and fluoride number were similar to reference values. PCR-SSCP analysis and following direct DNA sequencing analysis disclosed five missense or nonsense mutations in 11 individuals: 2 individuals with C to T mutation at codon 24 (Thr to Met), 2 individuals with G to C mutation at codon 365 (Gly to Arg), 3 individuals with T to A mutation at codon 330 (Leu to Ile), 1 individual with C to T mutation at codon 119 (Gln to stop) and 3 individuals with C to T mutation at codon 515 (Arg to Cys). For K variant, 3 homozygous individuals and 8 heterozygous individuals were determined. All of 14 individuals with low serum cholinesterase activity revealed genetic mutation in cholinesterase gene. Therefore, we should be aware of the existence of genetic mutations causing hypocholinesterasemia in laboratory diagnosis.