A stiffness matrix including both inertial and static axial force (prestress) effects is presented. The results of using this matrix to compute the natural frequencies of planar frames in the presence of prestress are presented. This matrix, when used in conjunction with a recently developed method for extracting eigenvalues of nonlinear eigenvalue problems, has proven to be very effective for computing natural frequencies and buckling loads of frames. It also allows the effect of prestress (static axial force) on natural frequencies to be determined.