  Fluctuations of observables as functions of time, or "fluctuation patterns",
are studied in a chaotic microscopically reversible system that has
irreversibly reached a nonequilibrium stationary state. Supposing that during a
certain, long enough, time interval the average entropy creation rate has a
value $s$ and that during another time interval of the same length it has value
$-s$ then we show that the relative probabilities of fluctuation patterns in
the first time interval are the same as those of the reversed patterns in the
second time interval. The system is ``conditionally reversible'' or
irreversibility in a reversible system is "driven" by the entropy creation:
while a very rare fluctuation happens to change the sign of the entropy
creation rate it also happens that the time reversed fluctuations of all other
observables acquire the same relative probability of the corresponding
fluctuations in presence of normal entropy creation. A mathematical proof is
sketched.
