PurposeSmall cell carcinoma of the ovary, hypercalcemic type (SCCOHT) is an extremely rare, highly aggressive cancer (mean age of onset, 24 years). Nearly all cases are associated with somatic or germline pathogenic variants (GPVs) in SMARCA4. Early bilateral oophorectomy is recommended for unaffected females with a SMARCA4 GPV. However, the penetrance of SMARCA4 GPVs for SCCOHT is highly uncertain and subject to ascertainment bias.MethodsLeveraging the early-onset, sex-specific, highly morbid nature of SCCOHT, we hypothesized that the penetrance for SCCOHT could be quantified from the deficit in SMARCA4 GPVs in females compared with males in UK Biobank, a population cohort for which recruitment was restricted to those age 40 to 69. We also analyzed pedigrees ascertained internationally by the Montreal-based SCCOHT-SMARCA4 Registry.ResultsWe observed SMARCA4 GPVs in 8/210,182 (0.0038%) female and 18/179,210 (0.0100%) male participants in UK Biobank (P = .028), representing a male:female odds ratio of 2.64 (95% CI 1.09-7.02), implying a penetrance of 62% for SCCOHT (given the absence of other SMARCA4-related female-specific early morbid diseases). A deficit of GPVs in females in UK Biobank was also demonstrated for BRCA1 and TP53.ConclusionOur findings support bilateral oophorectomy in early adulthood as a rational choice for at-risk females with SMARCA4 GPVs.