In contrast to historical accounts of the boom and crash of the 1929 stock market, recent econometric studies have concluded that there were no bubbles in the American stock market over the past one hundred years. Examining the pricing of loans to stock brokers, we find information on the lenders' perceptions of the future course of stock prices in 1929. From this market, we extract an estimate of the bubble in stock prices. This bubble component contributes significantly to explain stock price behavior, even though standard cointegration tests suggest that there was no bubble in the market.