Kashgarian, M. (Univ. of Tennessee Medical Units, 62 S. Dunlap St., Memphis, Tenn. 38103) and J. E. Dunn, Jr. The duration of intraepithelial and preclinical squamous cell carcinoma of the uterine cervix. Amer. J. Epid., 1970, 92: 211–222.—The age-specific incidence and prevalence rates of intraepithelial and squamous cell carcinoma of the uterine cervix are presented. These rates were obtained from cytologic screening of 150, 000 white and Negro women with 308, 000 person-years of rescreening experience during a period of 10 years. Prevalence rates are derived from the initial cytology screening and incidence rates are derived from repeat cytologic screenings of women who were negative on initial screening and first rescreening. Average durations of intraepithelial and preclinical invasive carcinoma have been calculated from these rates. The prevalence age-specific rates of intraepithelial carcinoma are bimodal in distribution, the first and major peak being in the 30 to 44 range and a minor peak at age 60. Preclinical invasive cancer prevalence rates increase gradually to age 50 after which they rise precipitously and then decrease after age 60. These distributions suggest that around the menopause host barriers are broken down and intraepithelial carcinoma rapidly becomes invasive. The evolution of disease processes can be calculated since changes in prevalence with increasing years of age of the population are determined by positive or negative increments resulting from the differences between input and output incidence rates bounding the prevalence interval. The average duration of intraepithelial carcinoma is about 11 years in white women, the average duration being longest between the ages of 25 to 35 and being shorter in younger and older women. These average durations have wide ranges. Negro women have twice as much invasive cancer as white women and progression of the disease process is slightly more rapid in Negroes than in whites.