Applications requiring the containment of large quantities of hydrogen gas at pressures greater than 70 MPa are anticipated in the evolving hydrogen economy infrastructure. Since hydrogen can promote subcritical crack growth in low-alloy steels, it is uncertain whether ASME SA-372 steels currently approved for pressure vessels are suitable for hydrogen gas containment at high pressures. The objective of this study is to measure subcritical crack growth thresholds for low-alloy steels in high-pressure hydrogen gas using fracture mechanics methods. The study focuses on three variables known to affect hydrogen-assisted crack growth in low-alloy steels: yield strength, alloy composition, and gas pressure. The test matrix includes low-alloy steels having yield strengths from 600 to 900 MPa. In addition, the steels are processed by air melting or vacuum arc remelting to vary the concentrations of P and S. Finally, hydrogen gas pressures range from 35 to 100 MPa.