The Stroop color-word test was used to examine patterns of cognitive decline in Alzheimer-type dementia (ATD) and non-Alzheimer dementia. Slowing on color naming and word reading was observed, and was greater in moderate than in mild dementia subjects. However, error scores were unrelated to dementia type and severity. The Stroop interference effect, measured with reaction time, was high in individuals with mild ATD and mild non-Alzheimer dementia. In contrast, the more severely impaired ATD subjects showed less Stroop interference effect than mildly impaired subjects when the reaction time was adjusted for color naming performance. These findings are attributed to variation in speed-accuracy tradeoff for the patient groups due to differences in information processing deficits, linguistic impairment, and attitudes to errors. This study demonstrates the importance of partialling out underlying deficits for the understanding of complex cognitive processes in dementia.