The size frequency distributions of β-amyloid (Aβ) and prion protein (PrP^sc) deposits were studied in Alzheimer’s disease (AD) and the variant form of Creutzfeldt-Jakob disease (vCJD) respectively. All size distributions were unimodal and positively skewed. Aβ deposits reached a greater maximum size and their distributions were significantly less skewed than the PrP^sc deposits. All distributions were approximately log-normal in shape but only the diffuse PrP^sc deposits did not deviate significantly from a log-normal model. There were fewer larger classic Aβ deposits than predicted and the florid PrP^sc deposits occupied a more restricted size range than predicted by a log-normal model. Hence, Aβ deposits exhibit greater growth than the corresponding PrP^sc deposits. Surface diffusion may be particularly important in determining the growth of the diffuse PrP^sc deposits. In addition, there are factors limiting the maximum size of the Aβ and florid PrP^sc deposits.