Abstract Polarised neutrons and polarisation analysis have been used to determine the diffuse magnetic scattering from FeSi as a function of temperature. The variation of the scattering as a function of Q is indicative of spatial correlations of magnetic moment extending over several interatomic distances. At 100 K the correlations are predominantly antiferromagnetic in character, but become increasingly ferromagnetic with increasing temperature. It is the thermal variation of these correlations which gives the unusual temperature dependence of the static susceptibility. The amplitude of the moment, estimated by integrating of observed response over an inverse atomic volume, was found not to vary significantly with temperature. A quantitative interpretation of these results is given in terms of a model in which FeSi shows properties of a heavy Fermi liquid.