The mechanisms behind the load building capabilities of a hyperbranched polymer (HBP) in polyurethane (PU) foams have been investigated, using microscopy techniques and mechanical analyses. By broadening the traditional uniaxial compression characterization of PU foams to include combined shear deformations and compression behavior, an apparent Poisson ratio of the foam could be obtained in situ. The Poisson ratio as function of uniaxial compression ratio of the foam was thus studied for foams filled with Styrene Acrylonitrile (SAN) and foams containing HBP. Generally a window of deformation ratios could be defined in which the Poisson ratio was negative. The width of this window varied systematically with the SAN loading, where an increase in SAN particle loading resulted in a broadening of the negative Poisson ratio window. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2290-2298, 2009