Lead exposure has been found to affect cognitive function in several different populations. Whether chronic low-level environmental exposure to lead results in cognitive decline among adults has not been examined. The authors assessed the relation between biomarkers of lead exposure and change in Mini-Mental State Examination (MMSE) scores in the Normative Aging Study, a cohort of elderly US men. Bone lead was measured with K-shell x-ray fluorescence. A total of 466 men aged 67.4 (standard deviation, 6.6) years took the MMSE on two occasions that were an average of 3.5 (standard deviation, 1.1) years apart during the period 1993-2001 and had bone lead concentrations measured during the period 1991-2002. A one-interquartile range (20 microg/g of bone mineral) higher patella bone lead concentration was associated with a change in MMSE score of -0.24 (95% confidence interval: -0.44, -0.05) after adjustment for age, education, smoking, alcohol intake, and time between MMSE tests. This effect is approximately equivalent to that of aging 5 years in relation to the baseline MMSE score in study data. The association with tibia lead was weaker and that with blood lead was absent. The data suggest that higher patella bone lead levels, a marker of mobilizable accumulated lead burden, are associated with a steeper decline over time in performance on the MMSE test among nonoccupationally exposed elderly men.