Although the prevalence of dementia increases among people with severe/profound intellectual (and multiple) disabilities (SPI(M)D), dementia in people with SPI(M)D is not yet fully understood. Therefore, this study aimed to characterize the natural history of dementia in people with SPI(M)D, in particular, the prevalence and time of onset of dementia symptoms.An explorative retrospective review of clinical records was conducted for people with SPI(M)D without dementia (n = 103), with questionable dementia (n = 19), and with diagnosed dementia (n = 19). Presence and time of onset of symptoms were extracted and compared between groups.People with questionable dementia or diagnosed dementia had compared to people without dementia more symptoms regarding the cognitive, activities of daily living, behavioral/psychological, and motor domains. The most prevalent early symptoms were memory loss, declined walking skills, increased anxious, apathetic, and irritable behavior. Predictors for dementia were the number of cognitive, behavioral/psychological, and motor symptoms.These results contribute to enhance our understanding of dementia in people with SPI(M)D, which is essential for earlier recognizing and diagnosing dementia.