Robert Walser (1878-1956) is among the most prominent German-speaking writers born in Switzerland. His early writings are fascinating due to his intensive affectivity and oneiric experiences; his late work impresses through his idiosyncratic use of language and his micrographs. Due to a psychotic disease he stayed in Swiss Mental State Hospitals (Waldau and Herisau) throughout the final 27 years of his life. According to his case records Robert Walser suffered from a schizophrenic disorder (ICD-10) and from a combined sluggish/manneristic catatonia according to K. Leonhard. Walser's psychotic disorder was characterized by a chronic course with sharp-cut symptomatology with stiff postures, repetitive behaviour, movement mannerisms and omissions (manneristic component) complemented by loss of incentive, severe autism and persistent verbal hallucinations (speech-sluggish component). In the late stages his psychopathology affected the process of thinking and writing in a specific manner: his handwriting became illegibly small, and his train of thoughts did not get to the point. At age 54 he stopped writing when transferred from Waldau to Herisau, and subsequently, due to manneristic omission, he was never again able to restart literary writing. The analysis of Robert Walser's psychotic disease may contribute to a deeper understanding of his literary production, which influenced such classical German authors like Franz Kafka, Hermann Hesse and Robert Musil.