Theodor Heller first described a severe regression of adaptive function in normally developing children, something he termed dementia infantilis, over one 100 years ago. Dementia infantilis is most closely related to the modern diagnosis, childhood disintegrative disorder. We translate Heller's paper, Uber Dementia Infantilis, and discuss similarities in presentation between Heller's cases, and a group of children with childhood disintegrative disorder. In particular we discuss a prodromal period of affective dysregulation described by Heller, and also evident in our sample, but not previously described in any detail since the publication of Uber Dementia Infantilis.