The impact of uncertainty shocks on firm‐level economic activity depends on their origin in supply chains. Upstream (downstream) uncertainty from suppliers (customers) is associated with variability over future input (output) prices. Consequently, a real‐option production model with time‐to‐build suggests that only upstream uncertainty suppresses investment, since upstream (downstream) uncertainty affects the shorter (longer) run. Production network data show that upstream uncertainty negatively affects firm‐level outcomes. Conversely, downstream uncertainty affects firm‐level outcomes more weakly but positively. At the macro level, these two uncertainties oppositely predict aggregate growth and asset prices. Overall, downstream uncertainty has an expansionary effect, in contrast to other facets of uncertainty.