Lysine is an essential amino acid and is supplemented as rumen-protected lysine (RPL) in the diets of dairy cows. A systematic review of literature and meta-analysis conducted in this study showed that feeding RPL during postpartum increased milk yield. Increasing metabolizable lysine as percentage of metabolizable protein (LYSMP) from 6.5 to 8.5% linearly increased blood concentrations of lysine, yields of milk, fat-corrected milk, energy-corrected milk, milk fat, and milk true protein; however, an interaction was observed between LYSMP and metabolizable methionine as percentage of metabolizable protein for feed efficiency and blood concentrations of essential amino acids in lactating dairy cows.