The stress-birefringence relationship was examined for amorphous poly(ethylene terephthalate) films under uniaxial stretching in the temperature range 80-95 degrees C at various stretch speeds in order to study the effect of strain-induced crystallization. At high temperatures, the ordinary stress-optical rule (SOR) held well at low draw ratios; the birefringence was proportional to the stress. However, at a certain draw ratio : the birefringence started to increase more rapidly than the stress and the SOR did not hold thereafter. D.s.c. measurements for films quenched at various stages of stretching supported the breakdown of the SOR being due to strain-induced crystallization. At still higher draw ratios, the stress suddenly began to increase. This is attributed to the non-Gaussian nature of the highly extended chains. The stress-optical coefficient has to be assumed to decrease with stress in order to estimate the crystallinity from the birefringence of films under high stress. At very low temperatures or at very high stretch speeds, the SOR was invalid even at low draw ratios due to the contribution of glassy stress. Even for such cases, the onset of strain-induced crystallization may be determined from the stress-birefringence relationship provided that the crystallization starts at relatively high draw ratios where the contribution from the glassy stress is small.