The role of race/ethnicity in genetic predisposition of early-onset cancers can be estimated by comparing family-based cancer concordance rates among ethnic groups. We used linked California health registries to evaluate the relative cancer risks for first degree relatives of patients diagnosed between ages 0-26, and the relative risks of developing distinct second malignancies (SPMs). From 1989-2015, we identified 29,631 cancer patients and 62,863 healthy family members. Given probands with cancer, there were increased relative risks of any cancer for siblings and mothers [standardized incidence ratio (SIR)=3.32;95% confidence interval (CI):2.54-4.35;P<0.001)]and of SPMs (SIR=7.12;95%CI:5.46-9.28;P<0.001). Higher relative risk of any cancer in siblings and mothers (P=0.001) was observed for Latinos (SIR=3.36;95%CI:2.24-5.05) compared to non-Latino White subjects (SIR=2.60;95%CI:1.66-4.06). Latinos had higher relative risks in first degree relatives and higher SPM risk compared to non-Latino Whites for most cancers, supporting a need for increased attention to the genetics of early-onset cancer predisposition in Latinos.