This paper provides an empirical investigation into the relationship between uncertainty and ex ante U.S. labor contract durations over the period 1970 to 1995. Using a structural identification of aggregate demand and aggregate supply shocks, we find that desired contract durations are shorter during periods of heightened nominal or real uncertainty. This evidence supports the view that labor contract durations respond endogenously to the aggregate uncertainty prevailing at the time they are negotiated, but suggests that risk-sharing concerns are not paramount. The analysis also considers several measures of inflation uncertainty that have appeared in the literature. The results generally corroborate previous findings of an inverse relationship between desired contract durations and the level of inflation uncertainty.