ImportanceAlthough most ovarian masses in children and adolescents are benign, many are managed with oophorectomy, which may be unnecessary and can have lifelong negative effects on health.ObjectiveTo evaluate the ability of a consensus-based preoperative risk stratification algorithm to discriminate between benign and malignant ovarian pathology and decrease unnecessary oophorectomies.Design, Setting, and ParticipantsPre/post interventional study of a risk stratification algorithm in patients aged 6 to 21 years undergoing surgery for an ovarian mass in an inpatient setting in 11 children’s hospitals in the United States between August 2018 and January 2021, with 1-year follow-up.InterventionImplementation of a consensus-based, preoperative risk stratification algorithm with 6 months of preintervention assessment, 6 months of intervention adoption, and 18 months of intervention. The intervention adoption cohort was excluded from statistical comparisons.Main Outcomes and MeasuresUnnecessary oophorectomies, defined as oophorectomy for a benign ovarian neoplasm based on final pathology or mass resolution.ResultsA total of 519 patients with a median age of 15.1 (IQR, 13.0-16.8) years were included in 3 phases: 96 in the preintervention phase (median age, 15.4 [IQR, 13.4-17.2] years; 11.5% non-Hispanic Black; 68.8% non-Hispanic White); 105 in the adoption phase; and 318 in the intervention phase (median age, 15.0 [IQR, 12.9-16.6)] years; 13.8% non-Hispanic Black; 53.5% non-Hispanic White). Benign disease was present in 93 (96.9%) in the preintervention cohort and 298 (93.7%) in the intervention cohort. The percentage of unnecessary oophorectomies decreased from 16.1% (15/93) preintervention to 8.4% (25/298) during the intervention (absolute reduction, 7.7% [95% CI, 0.4%-15.9%]; P = .03). Algorithm test performance for identifying benign lesions in the intervention cohort resulted in a sensitivity of 91.6% (95% CI, 88.5%-94.8%), a specificity of 90.0% (95% CI, 76.9%-100%), a positive predictive value of 99.3% (95% CI, 98.3%-100%), and a negative predictive value of 41.9% (95% CI, 27.1%-56.6%). The proportion of misclassification in the intervention phase (malignant disease treated with ovary-sparing surgery) was 0.7%. Algorithm adherence during the intervention phase was 95.0%, with fidelity of 81.8%.Conclusions and RelevanceUnnecessary oophorectomies decreased with use of a preoperative risk stratification algorithm to identify lesions with a high likelihood of benign pathology that are appropriate for ovary-sparing surgery. Adoption of this algorithm might prevent unnecessary oophorectomy during adolescence and its lifelong consequences. Further studies are needed to determine barriers to algorithm adherence.