Objectives: To quantify the time course of white matter hyperintensities (WMH) and assess the association between progression and cognitive decline in non-demented octogenarians. Methods: From a Danish cohort of 698 people born in 1914, 26 participated in neuropsychological assessment (Wechsler adult intelligence scale) initiated at age 50, including cognitive testing and cerebral magnetic resonance imaging at the 80 and 85 year studies. WMH volumes were quantified and partial correlations were calculated between WMH volume change and decline in WAIS scores from 80 to 85. Results: Progression in WMH volume ranged from 0 ml to 20.7 ml, providing a median increase of 2.6 ml (range 0.1 to 20.7, p<0.001) and, with a mean time interval between scans of 3.8 years, a rate of progression of 0.63 (0 to 6.8) ml/year. WMH volume measures for the two hemispheres were highly correlated (r = 0.95) and did not differ significantly. Increase in WMH volume was correlated with a simultaneous decline in verbal IQ (r = −0.65, p = 0.001), while baseline WMH was associated with subsequent decline in performance subtests (digit symbol, r = −0.57, p = 0.02). Conclusions: The association between WMH and decline in essential cognitive abilities even in well preserved elderly people suggests that WMH should be regarded as a risk factor for cognitive impairment and dementia.