The Missouri Department of Transportation (MoDOT) has discovered and documented failures in several cantilever mast arms in the recent years. The failures were primarily by fatigue at the weld of the arm to the base plate attached to the mast. With over 6000 mast arms in service in Missouri, the failures raised concerns with the existing mast arm inventory and future mast arm design. This report presents findings from an effort to determine the cause of unexpected cracking in Missouri mast arms. Three causes of premature failure were investigated: the stress ranges experienced at the weld detail were higher than anticipated, the number of cycles experienced at the weld detail were larger than anticipated and/or the weld quality was less than expected. The results show that the main culprit for the premature fatigue failure of mast arms in Missouri can be attributed to poor weld quality. The new "fatigue-resistant" weld detail, without quality welding techniques, does not improve the situation. The loads and cycles of loads experienced by the mast arms are not necessarily critical if the weld is of high quality. Recommendations for possible solutions for existing in-service mast arms and for new mast arms are presented. The recommendations range from insuring weld quality to dampers on the mast arms.