We introduce a new formalism to describe the polarization signal of galaxy clusters on the whole sky. We show that a sparsely sampled, half-sky map of the cluster polarization at z 1 would allow us to better characterize the very large scale density fluctuations. While the horizon length is smaller in the past, two other competing effects significantly remove the contribution of the small scale fluctuations from the quadrupole polarization pattern at z 1. For the standard CDM universe with vanishing tensor mode, the quadrupole moment of the temperature anisotropy at z 0 is expected to have a 32% contribution from fluctuations on scales below 6:3 h ÿ1 Gpc. This percentage would be reduced to 2% level for the quadrupole moment of polarization pattern at z 1. A cluster polarization would shed light on the potentially anomalous features of the largest scale fluctuations. [4,5] or inflationary physics [see, e.g., [6] ], and at the same time, evidenced the importance of independent observational methods to probe the largest scale structure in the Universe.The low-l moments of the present temperature anisotropies are usually assumed to probe the largest scales, but in fact they carry information about perturbations on a fairly broad range of scales. In this Letter we point out that the polarization signal from high-redshift clusters could probe the largest scales with less contamination from intermediate ones.Light scattered off a free electron is polarized, if the electron sees a quadrupole anisotropy of its incident light. This effect, which causes, for example, the large-scale polarization signal in a reionized universe [7,8], is responsible for the major polarization signal in SunyaeveZeldovich (SZ) galaxy clusters. As first pointed out by [9], the polarization pattern of clusters at different redshifts may shed light on the quadrupole amplitude at earlier times and at different positions [see also [10] in relation to the low observed quadrupole moment]. The cluster polarization has also been investigated as a potential tool to determine the nature of dark energy that is sensitive to the integrated Sachs-Wolfe effect [10,11].In this Letter we develop a transparent formalism for the calculation of the relevant observable quantities of the cluster's polarization signal, and show that the polarization