BACKGROUND: Psychosis is a frequent non-motor symptom in Parkinson's disease (PD). Estimates of the frequency of Parkinsońs disease psychosis (PDP) vary widely. Knowledge about the frequency and phenomenology of psychosis in late-stage (LS) PD patients is limited.This study aimed to determine the frequency of psychosis in LSPD patients through clinical diagnostic interview (CDI) (gold standard), according to NINDS/NIMH diagnostic criteria for PDP. The secondary objectives were to characterize the phenomenology, to test selected instruments and assess their adequacy in comparison to CDI, and to assess the psychiatric comorbidities. METHODS: A cross-sectional study including LSPD patients (patients with ≥ 7 years from symptoms onset and Hoehn and Yahr scale score > 3 or a Schwab and England scale score < 50% in the ON condition) was conducted. Patients were subjected to psychiatric, neurological, and neuropsychological evaluations. Each patient was interviewed by a psychiatrist who performed a CDI. RESULTS: 92 LSPD patients were included. 55.4% experienced psychotic symptoms according to NINDS/NIMH diagnostic criteria for PDP. Hallucinations were present in 94.1% and delusions in 29.4% of the psychotic patients. Visual hallucinations were the most common (88.23%) psychotic symptom. 72.5% of LSPD patients with psychotic symptoms had at least one comorbid psychiatric diagnosis. Lower frequency of psychosis was found when the assessment was performed only through selected instruments rather than CDI. CONCLUSIONS: A high frequency (55.4%) of psychotic symptoms and comorbid psychiatric (72.5%) diagnosis were found in LSPD patients. The use of CDI, in addition to structured scales may increase the sensitivity of detecting psychotic symptoms.