We encountered a 67-year-old female with chronic neutrophilic leukemia (CNL). Cytogenetic study showed she had a deletion in the long arm of chromosome 20. This finding indicates that CNL, in this case, is a clonal disorder. Most CNL patients have normal karyotypes, and only four patients with cytogenetic abnormalities, including two cases who received chemotherapy before the cytogenetic abnormality was detected, have been reported. Four of those cases, including our case, had abnormalities in the long arm of chromosome 20. This locus may be associated with the development of CNL. To our knowledge, this is the first case with CNL who showed deletion of the long arm of chromosome 20 before treatment was started.