Reported associations between coffee or caffeine intake and benign breast disease, cancers, and cardiovascular diseases have generally been weak and inconsistent. The apparent discrepancies in these studies might be attributable to imprecision in the measurement of coffee and caffeine intake. A study of a random sample of 2,714 U.S. adults disclosed considerable misclassification of total caffeine intake and, to a lesser extent, coffee intake when the estimates were limited to only the number of cups of coffee consumed. Adjustment for the following factors is recommended: amount of caffeinated and decaffeinated coffee consumed both on weekdays and on weekends; the size of the container used; the method used to brew caffeinated coffee; and the amount of caffeine imbibed from tea and soft drinks. Intake of coffee varied markedly between seasons of the year and over time. Random misclassification of coffee and caffeine intake would have the effect of obscuring dose-response relationships to disease incidence.