A patient with an atypical clinical picture of 18p- syndrome is described. By the in situ hybridization technique we localized the chromosome 18-specific cloned repetitive sequence to metaphase chromosomes of the patient. The predominant hybridization of the probe was found in pericentromeric regions of homologous chromosomes 18. The amount of pericentromeric DNA measured by in situ hybridization differed between homologous chromosomes; and the number of radioactive grains was statistically greater in the normal chromosome 18 than in the aberrant chromosome 18p-. The results indicate that this probe may be useful in clinical cytogenetics for identification of aberrant chromosomes, localization of breakpoints, and studies of C-band DNA polymorphism of chromosome 18.