The dependence of thermodynamic properties of QCD on the number of quark fl
avours is investigated. Lattice results for the equation of state are prese
nted for 2, 2+1 and 3 quark flavours. The simulations have been performed w
ith the improved p4-staggered fermion and a Symanzik improved gluon action
on lattices of size 16(3) x 4 and 16(4).