The objective of the study was to evaluate supplementing concentrated brewer’s yeast in the ration of dairy cows on lactation performance. We hypothesized that diets containing a concentrated brewer’s yeast supplement would benefit feed efficiency and increase milk and component yields. Thirty-six Holstein cows (24 multiparous and 12 primiparous; DIM = 71.17 ± 16.42) were used in an 8-wk randomized complete block design experiment. Cows were blocked by milk yield, DIM, and parity. Treatments included (1) control with no yeast (CON), (2) a concentrated brewer’s yeast product (Y1), and (3) a commercial yeast product (Y2). Cows were fed a common TMR, except for yeast supplements (14.2 g/h/d), once daily at 0800h using the Calan Broadbent feeder system to determine daily individual DMI. Cows were housed in a free stall barn and milked 2×/d and all milk weights were recorded. One day each week milk samples were collected for compositional analysis. Body condition scores (BCS) and body weights were obtained each week. Blood for plasma urea nitrogen (PUN) analysis was taken during wk 7 and 8. Data were analyzed using MIXED procedures with repeated measures and means were compared using Tukey’s test. Dry matter intake was similar (24.2, 24.6 and 24.1 kg/d for CON, Y1, and Y2, respectively; SEM = 0.82; P = 0.88); but there was a week by treatment interaction (P < 0.01) with cows fed Y1 having greater DMI during wk 2, 3, 4 of the study. Milk production (34.6, 34.6, 33.2 kg/d; SEM = 0.82; P = 0.28), milk fat (1.32, 1.29, 1.29 kg/d; SEM = 0.068; P = 0.41), and protein (0.97, 0.96, 0.94 kg/d; SEM = 0.033; P = 0.84) yields and other components were similar (P > 0.05) among treatments. Feed efficiencies, calculated as energy corrected milk/DMI, were similar among treatments (1.51, 1.36, 1.51; SEM = 0.063; P = 0.15), but there was a treatment by week interaction (P < 0.01). A treatment effect for PUN was detected (16.86, 14.10, 16.15; SEM = 0.444; P < 0.01). No statistical significance was determined for BCS and body weights (P > 0.05). Yeast products maintained performance, rather than improving production as hypothesized.