There are a large number of helices in proteins that contain the C-delta-(HO)-O-...=C hydrogen bonds involving Pro. Ab initio and density functional methods at HF/6-31G*, B3LYP/6-31+G**, B3LYP/6-311+G**, and MP2/6-31+G** levels are used to determine the factors that may lead to the stabilization (destabilization) of these C-delta-(HO)-O-...=C hydrogen bonds. The calculations are performed for 16 helical models with their structures generated from the X-ray structures of proteins. The direct C-delta-H and O=C interaction is not stabilizing for the cases where the C-delta-(HO)-O-...=C hydrogen bond immediately precedes a peptide hydrogen bond between (i+1)NH and (i-3)C=O groups (here i is the Pro residue number). The direct C-delta-H and O=C interaction is generally stabilizing for the cases where this peptide hydrogen bond is absent. Therefore, the formation of the (i+1)NH...(i-3)C=O peptide hydrogen bond is likely to force the C-H and O=C moieties into less favorable positions for the interaction. The strength of the C-delta-(HO)-O-...=C hydrogen bonds is significantly enhanced by cooperative hydrogen bonding involving the peptide hydrogen bonds in the helices and the capping interactions at the N-termini. The enhancement can be as large as 3-5 kcal/mol. The results suggest that the stability of the C-(HO)-O-...=C hydrogen bond depends on its environment, and this factor should be taken into consideration in the discussion of the energetic contribution of the C-(HO)-O-...=C hydrogen bonds to the stability.