A large number of C-alpha-H center dot center dot center dot O contacts are present in transmembrane protein structures, but contribution of such interactions to protein stability is still not well understood. According to previous ab initio quantum calculations, the stabilization energy of a C-alpha-H center dot center dot center dot O contact is about 2-3 kcal/mol. However, experimental studies on two different C-alpha-H center dot center dot center dot O hydrogen bonds present in transmembrane proteins lead to conclusions that one contact is only weakly stabilizing and the other is not even stabilizing. We note that most previous computational studies were on optimized geometries of isolated molecules, but the experimental measurements were on those in the structural context of transmembrane proteins. In the present study, 263 C-alpha-H center dot center dot center dot O=C contacts in a-helical transmembrane proteins were extracted from X-ray crystal structures, and interaction energies were calculated with quantum mechanical methods. The average stabilization energy of a C-alpha-H center dot center dot center dot O=C interaction was computed to be 1.4 kcal/mol. About 13% of contacts were stabilizing by more than 3 kcal/mol, and about 11% were destabilizing. Analysis of the relationships between energy and structure revealed four interaction patterns: three types of attractive cases in which additional C-alpha-H center dot center dot center dot O or N-H center dot center dot center dot 0 contact is present and a type of repulsive case in which repulsion between two carbonyl oxygen atoms occur. Contribution of C-alpha-H center dot center dot center dot O=C contacts to protein stability is roughly estimated to be greater than 5 kcal/mol per helix pair for about 16% of transmembrane helices but for only 3% of soluble protein helices. The contribution would be larger if C-alpha-H center dot center dot center dot O contacts involving side chain oxygen were also considered.