  The quark-gluon to hadron transition affects the evolution of cosmological
perturbations. If the phase transition is first order, the sound speed vanishes
during the transition, and density perturbations fall freely. This distorts the
primordial Harrison-Zel'dovich spectrum of density fluctuations below the
Hubble scale at the transition. Peaks are produced, which grow at most linearly
in wavenumber, both for the hadron-photon-lepton fluid and for cold dark
matter. For cold dark matter which is kinetically decoupled well before the QCD
transition clumps of masses below $10^{-10} M_\odot$ are produced.
