Gamma-ray burst are thought to be produced by highly relativistic outflows. Althoughupper and lower limits for the outflow initial Lorentz factor $Gamma_0$ are available, observationalefforts to derive a direct determination of $Gamma_0$ have so far failed or provided ambiguousresults. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Gamma_0$which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburstmaterial share a comparable amount of energy.We now comment early-time observations of the near-infrared afterglows of GRB,060418 and GRB,060607Aperformed by the REM robotic telescope.For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz,$Gamma_0sim400$.