The objective of this study was to determine the effect of replacing on isonitrogenous (16% CP) and isoenergetic (NE_L = 1.50 Mcal/kg) basis soybean meal and corn grain with fava bean (17%, DM basis) in dairy cow diets on N utilization. For this purpose, 9 lactating cows (DIM = 121 ± 28; milk yield = 41.2 ± 5.6 kg) were used in a replicated 3 × 3 Latin square (38-d period; 14-d adaptation). Treatments were: control diet (CTL), rolled fava bean diet (RFB), and ground fava bean diet (GFB). Intake of N, N excretion (fecal and urinary), milk N secretion, and MUN were determined over 6 consecutive days while ruminal NH₃ concentration was determined over 2 consecutive days. Data were analyzed using the MIXED procedure (SAS) and differences between treatments and the control were declared significant (P ≤ 0.05) using Dunnett’s comparison test. Nitrogen intake averaged 659 g/d and was not affected by the inclusion of RFB or GFB in the diet. Ruminal NH₃ concentration was higher for cows fed RFB and GFB diets (11.8 and 11.3 mM, respectively) than for cows fed CTL diet (9.8 mM). In contrast, MUN was unaffected by dietary treatments (11.6 mg/dL). Excretion of N in feces and urine was similar among dietary treatments (222 and 224 g/d, on average, respectively). Milk N efficiency (milk N secretion as a proportion of N intake) was similar between cows fed the CTL diet (29.6%) and those fed the RFB diet (29.0%), but decreased when cows were fed the GFB diet (28.5%). Feeding cows the RFB diet or the CTL diet had no effect on retained N (g/d or as a proportion of N intake). However, retained N was numerically higher for cows fed the GFB diet compared with cows fed the CTL diet (31.9 vs. 19.3 g/d and 4.85 vs. 2.86%, respectively). Results from this study suggest that similar efficiency of N utilization can be achieved when RFB replaces soybean meal and corn grain in dairy cow diets on isonitrogenous and isoenergetic basis. However, milk N efficiency can be depressed if GFB is used in dairy cow diets. In conclusion, at 17% dietary inclusion, fava bean should be fed rolled rather than ground to avoid negative effects of N utilization in dairy cows.