<p>The head cover of a 100-MW hydroelectric power machine was broken by fatigue after 35 years of operation. Stress measurements on a unit identical to the failed unit together with the power generation history were used to obtain a stress history and perform a fatigue life prediction. The stress measurement shows that the shaft and the operating ring transfer loads to the head cover, resulting in significant fluctuating stresses, which produce fatigue. The maximum damage and the minimum predicted fatigue life were found at the point close to the operating ring. Additionally, significant corrosion occurred, which reduced the thickness of the plates, increased the roughness, and accelerated fatigue failure. The head covers of the three units must be repaired to improve the contact surfaces with the operating ring, and a periodic program of cleaning, painting, and nondestructive inspection should be implemented.</p>