We report a detailed study of a cohort of sporadic Creutzfeldt-Jakob disease (sCJD) VV1-2 type-mixed cases (valine homozygosity at codon 129 of the prion protein, PrP, gene harboring disease-related PrP, PrP^D, types 1 and 2). Overall, sCJDVV1-2 subjects showed mixed clinical and histopathological features, which often correlated with the relative amounts of the corresponding PrP^D type. However, type-specific phenotypic characteristics were only detected when the amount of the corresponding PrP^D type exceeded 20-25%. Overall, original features of types 1 (T1) and 2 (T2) in sCJDVV1 and -VV2, including rostrocaudal relative distribution and conformational indicators, were maintained in sCJDVV1-2 except for one of the two components of T1 identified by electrophoretic mobility as T1²¹. The T1²¹ conformational characteristics shifted in the presence of T2, inferring a conformational effect of PrP^D T2 on T1²¹. The prevalence of sCJDVV1-2 was 23% or 57% of all sCJDVV cases, depending on whether standard or highly sensitive type-detecting procedures were adopted. This study, together with previous data from sCJDMM1-2 (methionine homozygosity at PrP gene codon 129) establishes the type-mixed sCJD variants as an important component of sCJD, which cannot be identified with current non-tissue based diagnostic tests of prion disease.