Steam ingress into a gas-cooled fast reactor (GCFR) core may lead to reactivity effects that are undesirable from the point of view of reactor safety. Unfortunately, the amount of reactivity increase caused by a certain steam concentration is usually subject to considerable uncertainty, as has become evident by occasional comparisons between various laboratories for specific examples. Therefore, some time ago, a series of intentionally simple benchmarks were proposed in order to study in a systematic way the calculational uncertainty of the steam ingress reactivity arising essentially from differences in the nuclear data basis used at various laboratories. The analysis of corresponding results provided by laboratories in France, Germany, Japan, Switzerland, and the United States reveals that there still exist appreciable deviations in the predicted steam ingress reactivity effect. Due to the extensive cancellation of positive and negative contributions to this reactivity effect, the resulting net value is extremely sensitive to deviations in the nuclear data and calculational methods. Typical discrepancies for the calculated steam ingress reactivity observed within the framework of an international intercomparison are described, leading to the conclusion that further improvements in the nuclear data basis are desirable and the development and application of fairly refined calculational methods is mandatory to be able to predict the corresponding effect with sufficient reliability for related power reactor designs. In addition, measurements of equivalent reactivity effects should be continued in different critical assemblies to provide a broader experimental basis for the verification of the calculational tools. If further analytical work could be pursued, the Argonne National Laboratory experiment on the GCFR Phase II Steam Entry Effect might be the appropriate object to be studied and analyzed in detail, e.g., by a similar intercomparison effort, especially if the discrepancies existing at present in nuclear data bases could be removed or diminished to a tolerable level. Reasonable progress in these areas would increase the confidence attributed to calculations of the reactivity effect of the assumed entry of hydrogeneous material into the core of a fast power reactor.