The topics and diagnostic discussions have been analyzed for the clinicopathologic conferences of the Massachusetts General Hospital, which were published during 1922, 1942, and 1962. The general distribution of topics has remained similar through the years, and the diseases under survey have remained almost exclusively entities of morbid anatomy, with "answers" provided by morphologic evidence. The general accuracy of pre-mortem diagnosis has declined during the 40-year period, probably because increasingly difficult cases have been selected for the conferences, although a misplaced confidence in para-clinical tests may be a contributing factor. In the diagnosis of cardiovascular-respiratory problems, almost half of the errors were due to "inaccurate focus." Other types of diagnostic errors were due to omission, inappropriate exclusion, and extraneous addition.