The effect of uniaxial deformation and subsequent relaxation at ambient temperature on irreversible and reversible crystallization of homogeneous poly(ethylene-co-1-octene) with 38 mol % 1-octene melt-crystallized at 10 K min was explored by calorimetry, X-ray scattering, and Fourier transform infrared spectroscopy. At 298 K, the enthalpy-based crystallinity of annealed specimens increased irreversibly by stress-induced crystallization from initially 15% to a maximum of, at least, 19% when a permanent set of more than 200% was attained. The crystallinity increased by formation of crystals of pseudohexagonal structure at the expense of the amorphous polymer, and as a result of destruction of orthorhombic crystals. The stress-induced increase of crystallinity was accompanied by an increase in the apparent specific heat capacity from 2.44 to about 2.59 J g(-1) K-1, which corresponds to an increase of the total reversibility of crystallization from, at least, 0.10 to 0.17% K-1. The specific reversibility calculated for 100% crystallinity increased from 0.67 to 0.89% K-1 and points to a changed local equilibrium at the interface between the crystal and amorphous phases. The deformation resulted in typical changes of the phase structure and crystal morphology that involve orientation and destruction of crystals as well as the formation of fibrils. The effect of the decrease of the entropy of the strained melt on the reversibility of crystallization and melting is discussed.