Uric acid levels of adult male volunteers in a longitudinal study of human aging rose steadily between 1961 and 1978. In the 1,141 men with 3 serial physical examinations, who developed no diseases and who took no drugs known to affect uric acid levels, levels rose from means below 5.5 mg/dl in 1961--1963 to means above 6.5 mg/dl in 1975--1978. The best predictor of a longitudinal increase in uric acid level was a gain in weight, but this, and other significant predictors, explained only a small portion of the increase in this population. Preliminary data available from a fourth examination indicate that the rising trend has leveled off.