The urinary excretion of aluminum was measured in 23 welders before and after an exposure-free interval of 16 to 37 days. In addition, the concentration of aluminum in the air was measured at the work site of 16 aluminum welders on the same workday as the first urine sample was taken. The concentration of aluminum in the urine depended on both the level and duration of exposure. The postshift urinary concentration of aluminum prior to an exposure-free interval was mainly related to the current air concentration, whereas the urinary concentration of aluminum determined after the exposure-free interval was related to total exposure duration (years). Among welders exposed for less than 1 year, the half-time for urinary concentration was about 9 days whereas welders exposed for more than 10 years had half-times calculated to be 6 months or longer. The results indicate that aluminum is retained and stored in at least two functional compartments of the body and is eliminated from these compartments at different rates.