The fimbria is the most common site of early serous cancer (tubal intraepithelial carcinoma or STIC) in women with BRCA mutations (BRCA+). A candidate serous cancer precursor--the p53 signature--has been found in nonneoplastic secretory cells of the fimbria, suggesting serous carcinogenesis in the tube (SCAT). This study surveyed fallopian tubes from 3 populations to characterize the morphological and immunohistochemical correlates of SCAT. The SCAT sequence was defined by strong nuclear p53 staining and DNA damage (gamma-H2AX+) in secretory cells and subdivided morphologically by (1) degree of nuclear stratification, (2) proliferative index, and (3) degree of disorganized growth. Fallopian tubes from women without a current ovarian cancer, women with BRCA mutations, and women with a coexisting pelvic serous cancer were completely examined. p53 signatures exhibited cuboidal to pseudostratified, polarized p53+ epithelial segments with variable nuclear enlargement and a MiB1 index of 0% to 30%. Tubal intraepithelial carcinomas contained from single (uncommon) to multilayered, poorly polarized, uninterrupted neoplastic cell populations that completely displaced the normal mucosa; MiB1 index exceeded 45% and was usually more than 70%. An uncommon third category, p53-positive foci with features intermediate between p53 signatures and STICs, exhibited preserved epithelial polarity, pseudostratification, incomplete replacement of the adjacent normal ciliated cells, and a MiB1 index between 40% and 75%. Transitions from 1 category to another were documented. Combined with recent reports associating STICs with pelvic serous cancer, this continuum of epithelial change validates the SCAT sequence and the fimbrial secretory cell as the site of origin for many serous carcinomas.