The processing-speed theory of cognitive aging contends tha age-related declines in intellectual function reflect the consequences of age-related slowing of processing speed. Cross-sectional data support this assumption. The association between 4-year changes in processing speed and 4-year changes in fluid intelligence was examined with a sample of 417 older adults. Changes in processing speed correlated .53 with changes in fluid intelligence. The difference in the explanatory power of processing speed regarding age-related differences and age-related changes is discussed with reference to other longitudinal studies and methodological considerations.