By using an idea of C$u{a}$dariu and Radu [4], we prove the generalized Hyers-Ulam stability of the functional equation f(x + y,z - w) + f(x - y,z + w) = 2f(x, z) + 2f(y, w). The quadratic form $f;:;mathbb{R};{ imes};mathbb{R}{ ightarrow}mathbb{R}$ given by f(x, y) = $ax^2;+;by^2$ is a solution of the above functional equation.