  We discuss the prospects of performing high-order perturbative calculations
in systems characterized by a vanishing temperature but finite density. In
particular, we show that the determination of generic Feynman integrals
containing fermionic chemical potentials can be reduced to the evaluation of
three-dimensional phase space integrals over vacuum on-shell amplitudes - a
result reminiscent of a previously proposed "naive real-time formalism" for
vacuum diagrams. Applications of these rules are discussed in the context of
the thermodynamics of cold and dense QCD, where it is argued that they
facilitate an extension of the Equation of State of cold quark matter to higher
perturbative orders.
