Although there is an increased risk of schizophrenia-spectrum psychoses (SSP) in people with intellectual disability (ID), there is a paucity of research evidence into clinical presentation of the disorder in comparison with research into SSP in people without ID. Aims The aims of the study were to compare clinical, functional, and social factors in patients with mild ID (ICD-10: F70) and SSP (ICD-10: F209) attending a specialist mental health service for people with ID, with a control group of patients without ID but with SSP attending a generic adult mental health (GAMH) outpatient clinic. A total of 106 patients with SSP (53 with ID and 53 from GAMH) were assessed on psychopathological symptoms, functioning scales and quality of life. They were compared using chi-squared and regression analysis where appropriate. People with ID and SSP appear to be more debilitated by the co-occurring disorder than those with the same disorder but without ID. Increases in observable psychopathology and negative schizophrenic symptoms, and decreased functional abilities were observed in the group with ID when compared to the GAMH group. The clinical implications of these findings are discussed.