  We evaluate microscopically the excitation spectrum of the 3He impurity in
liquid 4He at T=0 and compare it with the experimental curve at the equilibrium
density. The adopted correlated basis perturbative scheme includes up to two
independent phonons, intermediate correlated states and the correlation
operator is built up with two- and three-body correlation functions. The
experimental spectrum is well described by the theory along all the available
momentum range. A marked deviation from the simple Landau-Pomeranchuck,
quadratic behavior is found and the momentum dependent effective mass of the
impurity increases of about 50% at q around 1.7 A^-1 respect to its q=0 value.
No signature of roton-like structures is found.
