Background: Recent evidence suggests that cross-domain variability in cognition may be related to subsequent cognitive decline beyond mean performance levels in cognitive tasks. Objectives: To examine age-related changes in cross-domain variability across cognitive task performance in very old nursing home residents in contrast to community-dwelling older adults. To explore the relationship between cross-domain variability in cognition and functional disability in very old age. Methods: 204 very old (82.00 ± 8.51 years) residents from the Jewish Home and Hospital, Bronx, N.Y., and 376 community-dwelling older adults of similar age (86.75 ± 5.84 years) were tested on a cognitive battery. Cross-domain variability scores were computed across the cognitive tasks. Functional disability measures were derived from the CDR (Clinical Dementia Rating) Scale. Results: Whereas oldest old community-dwelling adults showed a decrease in variability across domains with age, variability increased with age in nursing home residents, irrespective of the level of cognitive performance. Cross-domain variability was associated with functional disability beyond the effects of age, gender, education, dementia status, residential status, and level of cognitive performance. Conclusion: Findings suggest that cross-domain variability in cognition is related to functional decline with age. Cross-domain variability in cognition may be a prominent predictor for the development of functional decline in very old adults.