Research published in the International Journal of Biological Sciences looks at the effects of feeding three strains of genetically-modified corn -- two of which produce Bacillus thuringiensis derived pesticides ("Bt"), and one of which is "Roundup ready", meaning that it contains derivatives of this herbicide -- to rats.
Looking at data that was actually provided by Monsanto (though in some cases, only after disclosure was mandated by courts), they found that
...in the three GM maize varieties that formed the basis of this investigation, new side effects linked to the consumption of these cereals were revealed, which were sex- and often dose-dependent. Effects were mostly concentrated in kidney and liver function, the two major diet detoxification organs, but in detail differed with each GM type. In addition, some effects on heart, adrenal, spleen and blood cells were also frequently noted. As there normally exists sex differences in liver and kidney metabolism, the highly statistically significant disturbances in the function of these organs, seen between male and female rats, cannot be dismissed as biologically insignificant as has been proposed by others. We therefore conclude that our data strongly suggests that these GM maize varieties induce a state of hepatorenal toxicity. This can be due to the new pesticides (herbicide or insecticide) present specifically in each type of GM maize, although unintended metabolic effects due to the mutagenic properties of the GM transformation process cannot be excluded. All three GM maize varieties contain a distinctly different pesticide residue associated with their particular GM event (glyphosate and AMPA in NK 603, modified Cry1Ab in MON 810, modified Cry3Bb1 in MON 863). These substances have never before been an integral part of the human or animal diet and therefore their health consequences for those who consume them, especially over long time periods are currently unknown.
Monsanto, of course, being one of the finest examples of pure concentrated evil on the planet, looked at this same data and applied weaker statistical methods to say that everything is hunky-dory.