We present a theoretical study of a mixed adenine-thymine crystal complex of trans-alpha(2) Pt(II) (alpha = CH3NH2) with Watson-Crick orientation of the bases, experimentally studied by Krizanovic et al. (J. Am. Chem. Soc. 1993, 115, 5538). Calculations are based on density functional theory with gradient-corrected exchange correlation functionals. The results for the structural parameters compare well with X-ray data. The electronic structure near the gap exhibits the characteristic features of a square planar platinum(II) complex with pi-acceptor ligands. Comparison with parallel calculations of the nonmetalated base pair helps clarify the role of the trans-alpha(2) Pt(II) moiety.