In the era of the serous tubal intraepithelial carcinoma hypothesis, investigation continues as to what proportions of high-grade serous tubo-ovarian carcinomas originate in the distal fallopian tube versus in the ovary. In this retrospective cohort study of 118,619 patients with high-grade serous tubo-ovarian carcinoma identified in the Commission-on-Cancer's National Cancer Database from 2004 to 2021, a diagnosis shift from high-grade serous ovarian carcinoma to high-grade serous fallopian tubal carcinoma occurred from 2004 to 2018 that the proportional distribution of high-grade serous fallopian tubal carcinoma increased 6.1-fold from 4.5% in 2004 to 27.6% in 2018 (p-trend <
.001). This rapid diagnosis shift from high-grade serous ovarian carcinoma to high-grade serous fallopian tubal carcinoma reached a plateau at 2018, followed by steady proportional distribution of high-grade serous fallopian tubal carcinoma among the high-grade serous tubo-ovarian carcinomas for 4 consecutive years (27.6% in 2018 to 28.0% in 2021, p-trend = .801). The average rate of tubal carcinomas during this post-plateau period was 27.7%. In conclusion, the diagnosis shift in the primary site of high-grade serous tubo-ovarian carcinoma from the ovary to the fallopian tube may have ended in the late 2010s. After the implementation of College of American Pathologists diagnosis criteria, 1 in 3 to 4 high-grade serous tubo-ovarian carcinomas were classified as of fallopian tube origin.