We have measured the momentum distribution and renormalization factor ZkF in liquid and solid lithium by high-resolution Compton scattering. High-resolution data over a wide momentum range exhibit a clear feature of the renormalization and a sharp drop of momentum densities at the Fermi momentum kF. These results are compared with those computed by quantum Monte Carlo simulation performed both on a disordered crystal and a liquid exhibiting very good agreement. Asymptotic behavior of the experimental and theoretical momentum distributions are examined to estimate ZkF. The experimentally obtained ZkF=0.43+0.11−0.01 for liquid Li and 0.54+0.11−0.02 for solid Li are in good agreement with theoretical results of 0.54±0.01 and 0.64±0.01, respectively.