The apparent contradiction between the absence of an effect of 325 mg aspirin taken every other day on the incidence of colorectal cancer in a randomized controlled trial and the consistent inverse association between aspirin and other non-steroidal anti-inflammatory drugs and colorectal cancer and polyps in observational epidemiologic studies is discussed. It is noted that the randomized trial of aspirin may have limited external validity. Information is needed on the effects of timing, duration and quantity of dose, and the effects of other non-steroidal anti-inflammatory drugs, particularly in view of the complexity of balancing established and potential risks and benefits of a possible public health intervention. Mechanisms whereby these drugs could influence the development of colorectal neoplasia are considered.

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