We report the results of an ultrastructural study of Pick bodies (PB). A histogram constructed with the maximal width of each filamentous component in PB revealed a wide range of sizes among the filaments, in contrast to the unique composition of the paired helical filaments (PHF) seen in the neurofibrillary tangle of Alzheimer type (NFT-AT). Morphologically, three groups of filaments could be distinguished. The first group consisted of straight smooth-surfaced filaments of 10-14 nm diameter, which were presumably altered neurofilaments. The second one was of straight smooth-surfaced "tubules" of 15-22 nm diameter, smaller than normal microtubules. The third one was of PHF thought to be formed by a pair of filaments of the first group. The PHF found in PB differed from PHF of NFT-AT in the distance between crossovers, and rather resembled the loosely interwinding PHF reported in NFT of progressive supranuclear palsy.