The objective of this study was to investigate the effect of supplementing active-dry yeast (ADY) on performance of lactating dairy cattle. Forty Friesian × Holstein cows (30 DIM) were enrolled a random complete block study. Cows were housed at the Tree House Farm (Drayton Animal Health, United Kingdom) in individual pens bedded with wood shavings. Candidate cows were sorted by calving date and randomly allocated into 1 of 2 treatments, ADY (Vistacell, AB Vista, UK; 3g/d providing 60 × 10¹⁰ cfu) or control (carrier only). Cows had ad libitum access to water via individual drinkers and received ad libitum TMR once a day for 12 wk. The ADY was applied as a top-dress. The TMR consisted (DM basis) of maize silage (39%), grass silage (39%), soy hulls (5%), wheat DDGS (5%), and a protein mix containing minerals and vitamins. A sample of the TMR was collected weekly throughout the experiment, composited monthly, and submitted for chemical analysis. The TMR contained 36.2% DM, and 24.3% ADF, 40.1% NDF, 21.0% CP, 7.8% ash, and 16.1% starch on DM basis. Orts were measured 3 time per week for each cow and milk yield was recorded daily for each cow. A weekly milk sample was collected form each cow and submitted for milk component analysis. Data were summarized by week and analyzed using PROC MIXED of SAS (v. 9.4). The model included the fixed effects of treatment (control, ADY), week (1 to 12), block (1, 2) and their interactions. The model included a repeated measurement statement with cow as the subject. The supplementation of ADY increased (P = 0.03) average daily milk yield (34.3 ± ­ 0.24 kg/d vs. 35.0 ­ ± 0.24 kg/d, control vs. ADY; respectively). This increase in milk yield was associated with an increase (P = 0.007) in feed efficiency (1.97 ­ ± 0.017 vs. 2.03 ­ ± 0.017). Dry matter intake, fat (content and yield), and protein (content and yield) were not significantly different between treatments. The effect of week was significant for DMI and milk yield, indicating a change in these parameters over time, but none of these parameters had a significant interaction with week. In conclusion, ADY supplementation during early phase of production, improved milk yield and feed efficiency.