Chromosome studies in 10 patients with chronic granulocytic leukemia are reported. The patients examined varied from asymptomatic untreated cases to extensively treated cases of several years' duration in terminal myeloblastic crisis. In 9 of the 10 patients, a similar chromosome abnormality was found. One of the four smallest acrocentric chromosomes in the leukemic cells was markedly reduced in size, apparently due to the loss of approximately half of its longer arm. With the exception of this finding, no frequent or regular abnormality in either chromosome number or structure was observed. The findings demonstrate a specific, consistent chromosome change associated with a particular human neoplasm and suggest that it is the chromosome change which confers on the leukocytes their neoplastic character. Such an association has not yet been demonstrated in any other human tumor.