Heritable influences on cognitive functioning were investigated in a sample of 403 pairs of like-sex Danish twins aged 75 years and older. Twins completed the Mini-Mental State Examination and 3 other cognitive tests. Genetic factors accounted for 26-54% of the variance on these measures, with the balance being due to environmental factors that create differences rather than similarities among reared-together relatives. Deleting twins with severe cognitive impairment had little effect on the results, indicating that the heritability of cognitive functioning was not due entirely to genes affecting dementia. Neither age nor gender moderated twin similarity, and differential social contact could not account for correlation differences between monozygotic and dizygotic twins. These results replicate G. E. McClearn et al.'s (1997) study in indicating substantial genetic influences on late-life cognitive functioning.