The effect of past exposure to asbestos on natural killer (NK) cell number and activity is uncertain. We measured NK cell number and activity in 1052 retired asbestos workers without symptomatic lung disease, lung cancer, or mesothelioma and with a long latency period from exposure; results were compared with those for 100 healthy age-matched controls. The exposed workers showed a decreased NK cell activity and increased NK cell number, yielding a 10.8 higher odds ratio for low NK activity per cell compared with controls (95% confidence interval 6.4 to 18.4), which was due to both a decrease in NK cell activity and an increase in NK cell number. Asbestos exposure of 10 years or more increased the risk of low NK activity per cell. We conclude that exposure to asbestos is associated with diminished effectiveness of NK cells and a concomitant increase in the number of NK circulating cells.