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The synergistic effect of hydrogenation and plasticpredeformation (PPD) on the fracture toughness and fatigue crack growthresistance in the tempered 0.4-1Cr-1Ni steel was studied. In the no hydrogenatedstate, PPD of steel slightly decreases its fracture toughness and increasesconsiderably the fatigue crack growth resistance in the near thresholdregion of da/dN - ΔK diagram. Crack closure is responsible for the lattereffect. Hydrogenation of the material drastically decreases the fracturetoughness and intensifies the fatigue crack growth, especially in the middleregion of da/dN - ΔK diagram. The synergistic effect of PPD and hydrogenationof the investigated steel causes the maximum decrease of crackgrowth resistance, both under static and cyclic loading. The possible consequencesof such effect from the point of view of structural strength of drillpipe string are analyzed.