First of all, if you are truly pre-med then there is absolutely no reason why you need to be deciding this right now. First focus on getting into med school, then do well in med school, make sure you still think you want to do IM with subspecialty (you may end up finding that you love surgery or radiology or something else). Then once you get into IM residency really make the final decision of cards vs nephrology. You probably want to start doing some research your 4th year of med school, I would do cards research b/c that is by far the more difficult subspecialty to match in, I know people who have matched nephrology with no research. To answer your question...the demand for nephrology will only continue to increase as Americans continue to choose to be unhealthy, get diabetes and end up on dialysis (not trying to insinuate that every dialysis pt did this to themselves but certainly a fair number). I know where I live currently the market is sort of saturated but neighboring cities still have job openings so I guess it varies. Hard to say what it will be like in 10 years though when you would be looking for a job.