By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 101¯3 peak in the direction perpendicular to the [101¯0]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of ∼1.19° after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to ∼1.36°, indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt. © 2000 American Institute of Physics.