Until the beginning of the twentieth century, neurologists considered that mental disorders in the course of Parkinson's disease (PD) occurred in the terminal phases of the disease or were due to coincidental pathologies. Benjamin Ball (1834–1893), in 1881 and 1882, drew attention to the frequency of cognitive and depressive disorders in PD. In 1883, Victor Parant (1848–1924), referring to Ball's work, published the first detailed observation of a PD patient with dementia and psychotic symptoms. Parant was an alienist running a private clinic for mental diseases in Toulouse, France. One of his main interests was the question of the responsibility of the insane, and he was called upon as a forensic expert in several cases. In this context, Parant examined a man who had been suffering from PD for several years, and later developed concurrently severe cognitive impairment and psychotic disorders. The patient would meet modern criteria for PD-associated psychosis: he had multimodal hallucinations (visual, auditory and somatic), visual illusions, and paranoid delusions. He also reported unusual symptoms: supernumerary limbs and Alice in Wonderland syndrome. Parant forwarded the far-sighted hypothesis that cognitive and psychotic disorders were due to the extension of PD lesions within the brain. The unheralded work of Victor Parant should be recognized in the history of neuropsychiatry.