This study was conducted to assess the effect of exposure misclassification when coffee is used as a surrogate measure of caffeine exposure. Subjects were randomly selected from the telephone directories of four regional municipalities in southern Ontario, CANADA: Data on daily caffeine intake from foods, beverages, and medications were collected from June to November 1995 through self-administered, mailed questionnaires from 481 men and women aged 30-75 years. Although coffee was the main source of caffeine, cross-tabulations of exposure to coffee by total caffeine intake showed that assessment of coffee alone severely underestimated caffeine intake by at least one exposure level. A hypothetical 10-fold increase in risk was completely obscured when only coffee was used to estimate total caffeine intake. The results of this study suggest that measuring coffee instead of caffeine intake may contribute to a lack of positive findings in studies of coffee as a risk factor for disease occurrence, if in fact caffeine is the exposure of interest. On the other hand, measurement of coffee, tea, and cola soft drink intake in the present study appeared to approximate caffeine intake sufficiently and not affect risk estimates adversely.