Studies of elderly people have shown that scores on tests of cognitive function tend to be higher in those with larger head circumferences. One explanation for these findings is that optimal brain development in utero and in the first years of life may protect against cognitive decline in old age, though the relative importance of these two periods of brain growth is unclear. We assessed change in cognitive function over a 3.5-year period in 215 men and women aged 66-75 years whose head circumference had been recorded at birth and as adults. Cognitive function was tested in the initial study and at follow-up with the AH4 intelligence test and the Wechsler Logical Memory test. We found no associations between head circumference at birth and score on the cognitive function tests or change in score over time. However, people who had a larger head circumference as an adult gained significantly higher scores on the intelligence test on both testing occasions and were less likely to show a decline in memory performance over the follow-up period. People whose head circumference was in the top quarter of the distribution had an odds ratio for decline in immediate recall on the Logical Memory test of 0.2 (95% confidence interval 0.1-0.6) and an odds ratio for decline in delayed recall of 0.3 (95% confidence interval 0.1-0.9) compared with those whose head circumference was in the bottom quarter, after adjustment for age, sex and potential risk factors. These results suggest that brain development during infancy and early childhood is important in determining how well cognitive abilities are preserved in old age.