I've run across a few articles stating it increases prostate cancer risk in men. Google will find those by searching 'flaxseed prostate'.
however the bigger issue is that there is not DHA or EPA in flaxseed, just ALA, which the body has to convert into DHA and EPA at a horribly inefficient rate, that only gets more efficient as we get older, and some people are even less effiicient than 'normal' in the first place.
so, prostate cancer risk or not, its just a poor substitute for fish oil.