Two reliability studies were performed on a recently developed cognitive assessment battery for severe dementia. The method, the Modified Ordinal Scales of Psychological Development (M-OSPD), is based on the Piagetian developmental model of sensorimotor functions. Procedures have been adapted from this test battery, which was originally applied to infants and small children, for the assessment of remaining cognitive capacity in severe dementia. Two independent interrater reliability studies were conducted. In these studies, two different raters simulataneously evaluated patients with severe dementia. One interrater reliability study was performed in a nursing home setting (Study 1), and the other reliability study consisted of a sample of community-residing patients (Study 2). The Global Deterioration Scale and the Mini-Mental State Examination were used to assess dementia severity. Study 1 (N = 22) resulted in an intraclass correlation coefficient (ICC) of .99 (p < .01) for the M-OSPD total score. Study 2(N = 19) resulted in an ICC of .96 (p < .01) for the M-OSPD total score. The M-OSPD proved to be a reliable instrument in these studies. This cognitive assessment measure can provide meaningful information regarding the cognitive abilities of late-stage dementia patients. Until recently, these late-stage dementia patients had been considered untestable in studies that utilized conventional psychometric and mental status evaluation measures.