Don't be confused!!  There are polyurethane foam mattresses, visco-elastic foam mattresses, and latex foam mattresses, plus many combinations.  Polyurethane foam breaks down and gets brittle very quickly.  Visco-elastic foam (like the Temperpedic brand) breaks down relatively quickly, and besides, you 'fall into a hole' when you lie on it.  Only latex foam retains its support virtually indefinitely.  It stays flat when you lie on it - it doesn't 'hammock'.  I have had my Therapedic Pure Touch Latex Foam Mattress for five years, and it shows absolutely no signs of 'dipping in' where I lie...