Mode I propagation rates for small cracks in a hardened AISI 52100 bearing steel under cyclic hydraulic pressure loading have been measured. Stress intensities were generated by applying high pressure fluid at the mouth of the crack without external far-field loads. Effects of crack size, cyclic pressure range, ambient fluid viscosity, and frequency have been observed. Some test conditions produced da/dN values comparable to baseline growth rate data collected in air under uniaxial cyclic tension while others resulted in significantly reduced or no crack propagation. Reduced crack growth rates are probably due to a lack of full fluid penetration into the crack or incomplete expulsion of fluid from between the crack faces both of which reduce Delta K-eff. Test data are compared to predictions from the characteristic penetration time model of Hsia and Xu. Qualitative agreement is observed, but some simplifications in the model, for example, constant viscosity, would require modification before qualitative comparison with experimental data can be made. (C) 1997 Published by Elsevier Science Ltd.