AbstractWartime changes in flour milling have been estimated to have more than doubled the intake of cereal fiber in England and Wales, Ireland, and Switzerland for periods of at least 6 years. These circumstances were used to test the hypothesis that cereal fiber inhibits colon carcinogenesis.Age‐standardized rates for deaths attributed to bowel cancer were plotted from 1931 to the mid‐1970s for England and Wales, Ireland, Switzerland, New Zealand, the US, and Australia. The ratio of the mortality rate observed 11 to 15 years after the change in extraction rates to that predicted from the prewar trend correlated with the increase in estimated cereal fiber consumption. The slope of the regression line suggested that any protective effect from cereal fiber was more modest than that inferred from other studies. The disparity between studies would be less if cereal fiber inhibited more than one stage of the process of carcinogenesis. Trends in six other countries with insufficient data for quantitative analysis were generally consistent (with the exception of those for Norway). The limitations of historical cause‐of‐death registrations as measures of cancer onset rates and the multiplicity of other changes during wartime require that our results be interpreted with caution.