A direct correlation has been established between the spatial variation of spectral luminescence and the distribution of threading dislocations in a green-light-emitting InGaN quantum well structure grown on a sapphire substrate. Transmission electron microscopy and monochromatic cathodoluminescence images, taken from the same region, indicate that the nature of the quantum well emission is influenced by the microstructure of the underlying GaN. The microstructure is defined by threading dislocations that reflect a columnar structure with low-angle grain boundaries. A strong correlation is observed between this microstructure and the peak and low-energy portion of the quantum well luminescence, with threading dislocations as boundaries between bright and darker regions. The high-energy portion of the luminescence is localized and is generally complementary to the rest of the spectrum. © 2007 American Institute of Physics.