Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in 561 K air and water over a wide strain amplitude range. It was found that fatigue resistance of the steels was enhanced in high-temperature water relative to high-temperature air under the low strain amplitude conditions (<0.3%) or in the high cycle regime (>2 x 10(4) cycles), while it was remarkably degraded in high-temperature water under the higher strain amplitude conditions. Fatigue cracking and fractographic features suggested that effects of hydrogen be involved in the present corrosion fatigue process in high-temperature water. Possible environmentally assisted cracking mechanisms are discussed. (C) 2005 Springer Science + Business Media, Inc.