  Gamma-ray burst are thought to be produced by highly relativistic outflows.
Although upper and lower limits for the outflow initial Lorentz factor
$\Gamma_0$ are available, observational efforts to derive a direct
determination of $\Gamma_0$ have so far failed or provided ambiguous results.
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 circumburst material share a
comparable amount of energy. We now comment early-time observations of the
near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM
robotic telescope. For both events, the afterglow peak was singled out and
allowed us to determine the initial fireball Lorentz, $\Gamma_0\sim 400$.
