Survival from ovarian cancer is strongly dependent on the stage at diagnosis. Therefore, when confronted with a woman with an isolated adnexal mass, clinicians worry about missing the opportunity to detect cancer at an early stage. High-grade serous ovarian cancers account for 80% of ovarian cancer deaths, largely because of their tendency to be diagnosed at a late stage. Among adnexal masses, large size and the presence of solid areas on ultrasound examination have been found to be associated with cancer, but it is unclear whether these characteristics identify early-stage cases. This study aimed to evaluate the ultrasound findings associated with clinically detected early-stage high-grade serous ovarian cancer. This was a retrospective cohort study of women diagnosed with stage I or II high-grade serous ovarian or fallopian tube cancer measuring at least 1 cm at pathology from 2007 to 2017. Preoperative ultrasound examinations were independently reviewed by 3 radiologists. Adnexal masses were scored for size and volume; overall appearance; presence, thickness, and vascularity of septations; morphology and vascularity of other solid components; and degree of ascites. Characteristics were compared between masses of <5 cm and larger masses and between stage I and stage II cases. Interobserver variability was assessed. Among 111 women identified, 4 had bilateral ovarian involvement, for a total of 115 adnexal masses characterized by ultrasound examination. The mean age at diagnosis was 61.8 years (range, 42-91 years). The median mass size was 9.6 cm (range, 2.2-23.6 cm) with 87% of cases having a mass size of ≥5 cm. A mixed cystic and solid appearance was most common (77.4%), but a completely solid appearance was more frequently seen for tumors of <5 cm compared with larger tumors (26.7% vs 13.0%). Solid components other than septations were seen in 97.4% of cases. The characteristics of stage I and II cases were similar other than ascites, which was more commonly seen in stage II cases (18.0% vs 3.1%, respectively). Interobserver concordance was high for size and volume measurements (correlation coefficients, 0.96-0.99), with moderate agreement observed across the other ultrasound characteristics (Fleiss kappa, 0.45-0.58). In this community-based cohort, early-stage high-grade serous cancers rarely presented as masses of <5 cm or masses without solid components other than septations. Our findings provide additional support for the observation of small masses without solid areas on ultrasound examination.