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Abstract Background IDH1/2 mutated glioma has been associated with a germline risk variant, the rs55705857 G allele. The Utah Population Database (UPDB), a computerized genealogy of people in Utah, is a unique resource to evaluate cancer risk in related individuals. Methods One hundred and two individuals with IDH1/2 mutant or 1p/19q co-deleted glioma were genotyped and linked to the UPDB. DNA came from blood (21), tumor tissue (43), or both (38). We determined congruence between somatic and germline samples and estimated the relative risk for developing cancer to first and second-degree relatives of G and A allele carriers at rs55705857. Results Somatic (glioma) DNA had 85.7% sensitivity (CI 57.2–98.2%) and 95.8% specificity (CI 78.9–99.89%) for germline rs55705857 G allele. Forty-one patients were linked to pedigrees in the UPDB with at least three generations of data. First-degree relatives of rs55705857 G allele carriers were at significantly increased risk for developing cancer (RR = 1.72, p = 0.045, CI 1.02–2.94), and specifically for oligodendroglioma (RR = 57.61, p = 0.017, CI 2.96–320.98) or prostate cancer (RR = 4.10, p = 0.008, CI 1.62–9.58); relatives of individuals without the G allele were not at increased risk. Second-degree relatives of G allele carriers also had significantly increased risk for developing cancer (RR = 1.50, p = 0.007, CI 1.15–2.01). Conclusions Tumor DNA may approximate genotype at the rs55705857 locus. We confirmed this locus confers an increased risk of all cancers and especially of oligodendroglioma. No increased cancer or brain tumor risk is seen in family members of individuals without the high-risk G allele.