In this study, the elastic buckling strength of cubic open-cell foams subjected to uniaxial compression is investigated using the homogenization framework developed by the present authors (Ohno et al., JMPS 2002; Okumura et al., JMPS 2004). First of all, based on the framework, the microscopic bifurcation and macroscopic instability of cubic open-cell foams are numerically analyzed by performing finite element analysis. It is thus shown that long wavelength buckling is the primary mode and occurs just after the onset of macroscopic instability. Then, a solution for predicting the stress of long wavelength buckling is analytically derived from the onset condition of macroscopic instability. The validity of this analytical solution is demonstrated by the finite element results.