This study reports and analyzes findings of traditional clinicopathologic conferences (CPC) from Massachusetts General Hospital. By comparing the distribution of topics and the accuracy in making correct diagnoses over several decades, the authors explore one of the oldest forms of reasoning. The cases reviewed were classified both by clinical topic and error type to better understand the trends and patterns seen in diagnostic failures. The authors discuss the teaching role employed by CPC and the potential for computers to play an integral role in the diagnostic reasoning process.