Repeat interviews from 4,088 Iowa pesticide applicators participating in the Agricultural Health Study provided the opportunity to evaluate the reliability of self-reported information on pesticide use and various demographic and life-style factors. Self-completed questionnaires were administered 1 year apart when participants returned to county agricultural extension offices for pesticide certification or training. Percentage agreement for ever-/never-use of specific pesticides and application practices was quite high, generally ranging from 70% to more than 90%, and did not vary by age, educational level, or farm size. Agreement was lower (typically 50-60%) for duration, frequency, or decade of first use of specific pesticides. Level of agreement regarding pesticide use in this population is similar to that generally found for factors typically used in epidemiologic studies such as tobacco use and higher than typically reported for diet, physical activity, and medical conditions.