Chinese banks likely have more nonperforming loans (NPLs) than officially reported. Banks’ NPLs often deviate from Benford's law. As hidden NPLs earn no interest income, loan quality problems may erode the gross interest income of banks. Using stochastic frontier analysis, we estimate the interest income of a hypothetical profit-maximizing Chinese bank with no credit quality problems. Taking the deviation of actual interest income from the calculated efficient income, we then attempt to reveal the amount of hidden NPLs in Chinese banks. Our results uncover a substantial weakening in the quality of Chinese bank loan portfolios in 2016.