A statistic often encountered in various estimation problems is the conditional ensemble average of the time derivative of a random signal given the signal. It turns out that for a very large class of random signals this statistic is equal to zero. This is a rather surprising result and as far as can be determined has not been precisely stated and rigorously proven. A precise statement and a rigorous proof of this theorem is the subject of this paper. Our result is the following. Let

be a stationary random process possessing a mean-square derivative

. Then the conditional ensemble average

always vanishes.