The chromosome patterns of 124 cases of cervical intraepithelial neoplasia (dysplasia and carcinoma in situ) and 87 cases of invasive carcinoma of the cervix were examined with the direct squash technique. Diploid or near diploid modes occurred exclusively in the dysplasias. Heteroploidy occurred in both carcinoma in situ and invasive carcinoma but the majority of cases of carcinoma in situ exhibited near tetraploid modes whereas the majority of invasive carcinomas had modes in the near diploid range. These differences were significantly different (P<0.001), as were the differences in chromosome pattern in small cell and large cell lesions, both in situ and invasive. Chromosome patterns also showed some correlation with the types of abnormal mitoses in histologic section. A considerable number of cases of carcinoma in situ and invasive carcinoma did not exhibit a mode although modes were common in stage I and stage IV carcinoma. These results are discussed in relation to the progression of preinvasive lesions.