This paper studies the welfare effects of the relative-price variability arising from inflation. If customers and suppliers form long-term relationships, prices have an informational role: a potential customer uses current prices as signals of future prices. Inflation reduces the informativeness of current prices, causing customers to make costly mistakes about which relationships to enter. In addition, the reduced informativeness of prices makes demand less price elastic, thereby increasing markups. Both effects can be quantitatively significant at moderate inflation rates.