In the follow-up studies of dysplasia of the uterine cervix, the rates at which 691 cases of mild and 654 cases of severe dysplasia progressed to malignant lesions were, respectively, 0.3% (2 cases) and 17% (111 cases). A classification of the cases of severe dysplasia was made according to the degree of maturation of the lesion into immature, premature and mature dysplasia. The rates of progression to malignancy for these three types were 28.4%, 16.7% and 5.9%, respectively. In cases of carcinoma in situ progressing from immature dysplasia, 74.4% of such cases were of the small cell type, whereas in cases progressing from mature dysplasia, 85.7% were of the large cell type. The similarity in site of the lesion within the cervix was particularly notable between immature dysplasia and carcinoma in situ of the small cell type.