A comparison of diary and mail methodologies for a cohort of anglers who fished Lake Ontario was used to examine biases associated with 12-month recall from mail questionnaires. Significant differences in estimates were found between estimates reported by respondents in diaries (1992) versus mail questionnaires (1991) for number of days fished and fish consumption, but not for fishing expenditures and catch. After the data were adjusted for a decline in fishing on Lake Ontario between 1991 and 1992, it was found that angler-days were overestimated by 44–45% on the 12-month recall mail questionnaire. This percentage can serve as an initial estimate of a correction factor for future studies. Lower average annual fish consumption rates were reported in the diary year compared with the mail questionnaire year. However, because of the lower percentage of meals of sport-caught fish during the diary year and the knowledge that sportfishing declined in 1992 for Lake Ontario anglers, it is not clear what portion of the decline can be attributed to different factors. A rough estimate of 10% can be obtained by assuming that consumption of fish that were not sport caught was the same in both years and that anglers accurately reported the overall percentage of that consumption in 1991. Less avid anglers had a very small positive discrepancy between their mail (1991) and their diary (1992) estimates of fishing participation, whereas anglers who fished more frequently had a much larger positive discrepancy. With these data, the best mathematical procedure for describing that relationship involved regressing the square root of days fished in 1991 against days fished in 1992.