A model is presented that establishes a relationship between the pathogenicity of viroids and the conformation of a part of their molecules. The model is applicable to the viroid group composed by potato spindle tuber viroid, citrus exocortis viroid and chrysanthemum stunt viroid. It is assumed that viroids adopt in vivo a rod-like structure which essentially coincides with their so-called native structure in vitro. The viroid pathogenicity is associated with the conformation of a segment of the rod-like structure between bases 43–44 and 54–56, and particularly with the alterations in some specific points of the double-stranded structure of this segment. The model is consistent with the observation that viroids of this group, with a sequence homology of only 73%, induce in some hosts symptoms of similar severity, whereas isolates of the same viroid, with a sequence homology of 98% differ notably in their pathogenicity.