Avoiding pesticides in food is another of the most well-known reasons to eat organic food.
The USDA tests for pesticide residues on selected foods, using a fairly large number of samples. They find that the vast majority of non-organic ("conventional") fruits and vegetables contain pesticide residues, and that few organic fruits and vegetables do.
In 1994-1999, the USDA found at least one pesticide residue on 80% of conventional fresh fruits and 75% of conventional fresh vegetables. There were two or more pesticides on 45 % of the samples, and the average apple had three pesticides.
In 2002, the USDA found pesticide residue in 22% of conventional processed food samples.
The Organic Center analyzed these and other test results from a ten-year period. They found that 77% of conventional fruit, 63% of conventional vegetables, and only 18% of organic fruit and vegetables had pesticide residues. The conventional produce had levels of pesticide that were three to ten times higher, and was about ten times more likely to have residues of multiple pesticides.
The Organic Center discussion of this data is here: Minimizing Pesticide Dietary Exposure Through the Consumption of Organic Food
But does eating organic food significantly affect the amount of pesticide actually in our bodies?
The single most important study was conducted with children in the Seattle area. In the study, the metabolites, or breakdown products, of pesticides were measured in children's urine. Metabolites of two particularly dangerous agricultural pesticides were detected in all the children, without exception.
During the fifteen-day study, children ate their usual foods on days 1 through 5; they ate the exact organic equivalents of their usual fruit, vegetables, and processed foods on days 6 through 10; and they returned to their usual diet on days 11 through 15. On the days the children ate organic food, and only on those days, the pesticide metabolites became undetectable.
This is the original journal article: Organic Diets Significantly Lower Children's Dietary Exposure to Organophosphorus Pesticides