Journal article: What caused the 1929 crash of the New York Stock Exchange? This paper provides a quantitative study of liquidity in the 1929 crash of the NYSE. I evidence the crash was indeed a liquidity crisis due to the liquidation of brokers’ margin loans. Applying recent estimators of effective spreads and liquidity conditions from contemporary finance literature suggests a fourfold increase in spreads during the crash at the aggregate level. At the individual stock level, quoted bid-ask spreads suggest liquidity explains one-fifth of the variance in daily stock returns in the crash.