We derive the strong-stretching approximation from the full partition function for diblock copolymer melts. Using a perturbation expansion for the free energy, with (gamma N)(-1/3) as the expansion parameter, we calculate the leading corrections to the asymptotic chi N --> infinity limit for the domain period and the free energy. The leading corrections to the free energy scale as log chi N They arise from the entropy gain due to the wandering of chain ends over the domains and the entropy loss to localize the joints in the interfacial region.