  We perform an extrapolation from negative to positive flavour numbers of full
QCD lattice estimates of the $\eta'$ mass. The extrapolations are carried out
by keeping $\rho$ and $\pi$ masses at fixed values. We find an $\eta'$ -- $\pi$
splitting which shows a flavour dependence consistent with the Witten Veneziano
formula based on the $U(1)$ anomaly. The quantitative splitting is consistent
with the estimates made in the quenched approximation.
