Optical properties of laterally overgrown GaN hexagonal pyramids on (1 1 1) Si substrates are studied by cathodoluminescence (CL) spectroscopy and mapping techniques. The results are compared with structural properties obtained by scanning and transmission electron microscopic techniques. To clarify the origin of the bandedge and yellow-band emissions from the GaN pyramids, wavelength-resolved CL properties of normal and cleaved GaN pyramids are investigated in the top and/or cross-sectional view configurations. The cross-sectional view CL images for cleaved GaN pyramid samples show significant differences between the overgrown areas on top of the mask and the coherently grown regions over the windows. A precise reverse (identical) contrast between bandedge (yellow-band) emission intensity and threading dislocation density is observed by comparing the cross-sectional view CL and transmission electron microscopic images. It is demonstrated that a strong correlation exists between structural defects and optical properties in laterally overgrown GaN hexagonal pyramids. (C) 2002 Elsevier Science B.V. All rights reserved.