See This Thread over at PriusOnline. Besides, it's a good time to vacuum it out as the weather's warming up to make sure your AC is not running any harder than necessary so you can really see that nice summer Fuel Economy Boost. Coastaletech sells the replacement filters for $19 + S&H, that's about as cheap as I could find them. If someone knows how/where to get them cheaper please share. Also, John1701a has a very nice write up on how to find and replace. Probably more detail than most need, but if you're not very handy it may help. I checked mine today after seeing that thead. Had a few leaves and some other junk in there so I pulled out the shop vac and cleaned it up before sticking it back in. I figure it'll wait another 5k miles. Also, I always forget who, someone here has a nice Hepa filter he just cuts up to fit the spot and it works out to be much cheaper than getting the 'official' filter from Toyota. I haven't done that yet, but perhaps I should.