We report the discovery of possible overdensities of galaxies at z \sim 3.7 in Chandra Deep Field South ( CDF-S ) . These overdensities are identified from a photometric redshift-selected sample , and the BVz -selected sample . One overdensity is identified in the proximity of two active galactic nuclei and Lyman break galaxies at z = 3.66 and z = 3.70 at 7 \sigma significance level . The other overdensity is less significant . It is identified around six z _ { spec } \simeq 3.6 galaxies at 3 \sigma significance level . The line-of-sight velocity dispersions of these overdensities are found to be \sigma _ { v } \simeq 500 - 800 km s ^ { -1 } , comparable to the velocity dispersions of clusters of galaxies today . Through spectral energy distribution fitting , we find \sim 15 massive galaxies with M \gtrsim 10 ^ { 11 } M _ { \odot } around the z \simeq 3.7 overdensity . The mass of the z \simeq 3.7 overdensity is found to be a few \times 10 ^ { 14 } M _ { \odot } . Our result suggests that high-redshift overdense regions can be found in a supposedly blank field , and that the emergence of massive structures can be traced back to redshifts as high as z \sim 3.7 .