Estimating the future frequency of extreme hydrologic events remains a common problem. Present procedures use hydrologic records to estimate future frequencies of occurrence, assuming that hydrologic regimes are stationary, despite contrary evidence in South America. This paper is concerned with estimating frequencies of hydrologic extremes under conditions for which trends in hydrologic regime have been detected, and where it is not unreasonable to suppose that such trends will continue into the near future. Expressions are given (i) for the probability that a given critical extreme Qcrit (a critical flood discharge, for example) will occur at least once in the next K years, when trend has been found in the sequence of annual extreme values; (ii) for the probability that a peak flood equal to or larger than Qcrit will occur at least once during a period <0,T) where the 'peaks-over-threshold' method is used.