Hand-grip force was measured in 63 professional lumberjacks in 1978 and again in 1980. Lumberjacks with vibration-induced white fingers (VWF) had lost 21% of their muscle force during the two years. Lumberjacks with no hand-arm symptoms had lost 5% of their muscle force in the same time period. Lumberjacks with subjectively diminished hand muscle force had a slight increase in muscle force during the follow-up time. These results suggest that long-term exposure to vibration causes a decrease in muscle force.