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We calculate the Cosmic Microwave Background anisotropy bispectrum on largeangular scales in the absence of primordial non-Gaussianities, assuming exactmatter dominance and extending at second order the classic Sachs-Wolfe result\delta T/T=\Phi/3. The calculation is done in Poisson gauge. Besides intrinsiccontributions calculated at last scattering, one must consider integratedeffects. These are associated to lensing, and to the time dependence of thepotentials (Rees-Sciama) and of the vector and tensor components of the metricgenerated at second order. The bispectrum is explicitly computed in theflat-sky approximation. It scales as l^(-4) in the scale invariant limit andthe shape dependence of its various contributions is represented in 3d plots.Although all the contributions to the bispectrum are parametrically of the sameorder, the full bispectrum is dominated by lensing. In the squeezed limit itcorresponds to f_NL^local = -1/6 - cos(2 \theta), where \theta is the anglebetween the short and the long modes; the angle dependent contribution comesfrom lensing. In the equilateral limit it corresponds to f_NL^equil ~ 3.13.

Sachs-Wolfe at second order: the CMB bispectrum on large angular scales