Geometries and energies of the reactants, complexes, and transition states for the reactions of lithium aluminum hydride with formaldehyde and cyclohexanone were obtained using ab initio and density functional (Becke3LYP/6-31G**) molecular orbital calculations. Two pathways for reaction with formaldehyde and four transition states corresponding to axial and equatorial attack at cyclohexanone were located. The transition state structures had reactant-like geometries. Predicted stereoselectivity of the reduction of cyclohexanone strongly favors axial approach of hydrogen, in agreement with experimental data. Analysis of the transition state structures suggests that electronic effects are more important than torsional effects in controlling stereoselectivity.