The role of microhydration in structural adjustments of the AT-tract in B-DNA was studied at the B97-D/def2-SV(P) level. The (dA:dT)(5) complexes with 10 water molecules in minor and 15 water molecules in major grooves were studied. The obtained network of hydrogen bonds revealed the dependence between the groove width and the types of water patterns. In the minor groove, the following patterns were observed: interstrand one-water bridges similar to that of the Dickerson water spine and interstrand two-water bridges. The network of structural waters in the major groove is more diverse than that in the minor groove, which agrees with crystallographic data. As the major groove is wider, it is enriched by water molecules forming two- and three-water bridges. Results suggest the nucleobasewater interactions in both grooves prevent AT-tract twisting and its collapse along the minor groove. Whereby, a helix structure with narrow minor and wide major grooves is formed. The structural waters affect the polynucleotide conformation so that it becomes similar to poly(dA) center dot poly(dT) in fibers and acquires features of the A-tracts in DNA in solution. We suggest that formation of specific water patterns in both grooves is the factor responsible for stabilization of A-tracts with a narrowed minor groove, leading in turn to their strong intrinsic bending in DNA.