Observations in high bandwidth graded-index plastic optical fibers (GI POFs) are reported of strong mode coupling and its characteristic square root-like fiber length dependence of the optical pulse broadening. We have further found that the measured refractive index profile is not parabolic, but consists of two markedly different regions. Analysis of the index profile reveals that strong mode coupling increases the GI POF bandwidth from its profile-determined value of 0.43 GHz to its measured value of 3.0 GHz for 100 m. ©1997 American Institute of Physics.