The Mann-Kendall test, used to detect a trend in a time series, yields an incorrect (too large) rejection rate when applied to an autocorrelated series with no trend. Prewhitening corrects this Situation, but reduces the power of the test when a trend exists. A simulation study is performed to determine when prewhitening can be applied with no real loss of power. It is found that, in general, prewhitening Should be avoided when the test has a high power, i.e. when the coefficient of variation is very low, the slope of trend is high, and the sample size is large. In other cases, prewhitening will prevent the false detection of a non-existing trend, Without a significant power loss in identifying a trend that exists.