Most of the data used to study the association between MUN and protein efficiency was from cows fed excessive protein. Few studies examined how the relationship between MUN and protein efficiency was affected by dietary protein content and lactation stage. In this study, 69 Holstein cows (in 8 experiments) were fed diets containing sufficient and deficient protein in both mid and late lactation. In each lactation stage, cows were included in the crossover experiment with two 28-d treatment periods. The diets contained 18% or 14% CP in mid lactation, and 16% or 12% CP in late lactation. All 4 diets contained sufficient RDP for rumen function with expeller soybean meal added to achieve the high protein diets. Cows were milked 2× daily; intake and MY were recorded daily. Milk composition was measured during 4 consecutive milkings weekly and BW was measured 3× weekly. Protein efficiency was calculated as dietary protein captured in milk protein (milk protein efficiency, MPE), and dietary protein captured in milk protein and body tissues (gross protein efficiency, GPE), respectively. MUN, MPE and GPE were calculated for each cow in mid lactation averaged across both diets to compare against values in late lactation averaged across both diets and for the high-protein diets compared with the low protein diets averaged across both stages of lactation. Pearson correlation coefficients among MUN, MPE and GPE across lactation stage and dietary protein content were calculated after accounting for effects of parity, cohort and experiment. MUN of individual cows was repeatable across dietary protein content (r = 0.8, P < 0.05) and lactation stage (r = 0.5, P < 0.05). MPE was repeatable across dietary protein content (r = 0.8, P < 0.05) but not lactation stage (r = 0.2, P = 0.2). GPE was repeatable across dietary protein content (r = 0.7, P < 0.05) but not lactation stage (r = 0.2, P = 0.2). On average, protein efficiency was greatest for low-protein diets that produced lowest MUN regardless of lactation stage; however, protein efficiency of individual cows was not correlated with MUN within diets. In conclusion, some cows consistently have lower MUN regardless of diets or stage of lactation; however, they are not necessarily using protein more efficiently.