The strain energies in biphenylene and some of its bridged derivatives have been calculated by using a high-level ab initio method (G3MP2//B3LYP-6-31G*) in order to resolve the long-standing discrepancy in the previously reported values. The results show that the strain energy of biphenylene is approximately 222 kJ/mol, which is lower than the previously reported values. The strain energies in 1,8-bridged biphenylene derivatives were also calculated and turned out to be larger than previously suggested. Our results highlight the need to use thermodynamic information in conjunction with structural data when discussing strain energy or molecular deformation.