The root cause of a series of similar failures in SA-213 T91 Superheater tubes of an Heat Recovery Steam Generator (HRSG) is investigated using a combination of engineering analysis and review of process data. The HRSG at the Combined Cycle Gas Turbine (CCGT) Power Plant power plant in question had suffered from frequent tube-to-header fatigue failures over the past 10 years. Metallurgical analyses had never identified any sign of creep damage in, or near, any of the failure locations. Recently, the Gas Turbine (GT) exhaust gas flow pattern upstream of the SH tubes changed slightly. Subsequently there were a large number of HPSH tube to header failures (> 10) on one side of the gas duct. Metallurgical analysis showed that the tube-to-header welds failed by creep-fatigue damage; analyses of tubes from the left-hand side of the boiler did not show any signs of similar damage being present. Further investigation confirmed that the root cause was identified as higher temperatures resulting from small changes in the GT outlet flow pattern.