Direct marrow cytogenetic preparations were examined in 14 cases of anemia associated with deficiency in vitamin B12 and/or folate. Seven cases showed distinct alterations in chromosome structure consisting of increased chromosome breakage, incomplete chromosome contraction and centromere spreading. These abnormalities were not present after vitamin replacement and were generally less distinct in cases with major medical complications or with only mild megaloblastic changes. No distinct aberrations in chromosome number or karyotype were found. The cytogenetic changes observed seem compatible with current concepts of megaloblastic cell division and with the role of folate and vitamin B12 in DNA metabolism.