Abstract : We provide a model-independent determination of the quantity B_0(m_d-m_u). Our approach rests only on chiral symmetry and data from the decay of the eta into three neutral pions. Since the low-energy prediction at next-to-leading order fails to reproduce the experimental results, we keep the strong interaction correction as an unknown parameter. As a first step, we relate this parameter to the quark mass difference using data from the Dalitz plot. A similar relation is obtained using data from the decay width. Combining both relations we obtain B_0(m_d-m_u)=(4495 +/- 440) MeV^2. We use the preceding value to estimate the size of the violation of Dashen's theorem and the quark mass ratio m_u/m_d.