To estimate the relative effects of coexisting nondementia illnesses on the probability of functional disability, depending on the presence of cognitive impairment or dementia, we used data from the baseline case-control assessment of a longitudinal study of aging and dementia. Our study included 668 subjects (345 nondemented, 98 cognitively impaired, and 225 demented), aged 75 and older. Demented subjects had greater disability prevalence on all specific instrumental activities of daily living (IADL) and activities of daily living (ADL) items than cognitively impaired subjects who, in turn, had greater disability than nondemented subjects. Somatic illnesses were found to be associated with particular tasks in item specific models; decreasing MMSE was strongly associated with the probability of IADL and ADL disability, which increased dramatically in the presence of somatic illnesses among cognitively impaired as well as demented subjects. Attention to illnesses among cognitively impaired and demented people may shed light on remediable factors crucial to their daily functioning.