An investigation to determine the magnitude of geometric imperfections induced during the fabrication of welded steel box girders is reported for nine highway bridges constructed in Canada. Measurement data were collected to compare the significance of dead load on initial imperfections obtained following fabrication. Existing bridges were found to have out-of-plane deviations of plate panels about 10% higher than unloaded bridge values. A statistical interpretation of measurements of initial plate imperfections and out-of-straightness of stiffeners culminates in tolerance levels, deemed to be appropriate for Canadian-built box girders, and well within the capability of the fabricating industry to achieve. Key words: box girder, bridges, field measurements, geometric imperfections, plate panels, stiffeners, structural steel, tolerances.