The objective of the current research was to examine the association of feeding NutriTek, a Saccharomyces cerevisiae fermentation product (Diamond V, Cedar Rapids, IA) in close-up and lactating cows with udder health. Production records and clinical mastitis event data were collected from 25 herds from 5 regions of the US using back-up files from herd DairyComp305 collected from June to September 2017. This provided production and event records that ranged from 400 to 890 d following NutriTek feeding. Herds ranged in 305-d milk production from 7,800 to 12,700 kg, and the number of cows in the adult herd ranged from 880 to 10,727 cows. Test day records were extracted for herds that recorded linear scores (LS) by cow, event data for mastitis, date of calving, and date of herd removal for an equal period of time before and after feeding NutriTek. Fifteen herds had test day records with LS data and all 25 herds had event data. A total of 1,444,450 production records were available for analysis: 702,449 records were for the period before, and 742,000 records post NutriTek feeding. Lactation records were categorized by herd, cow, lactation number (where available), date fresh, and date of test. Data for LS were analyzed using SAS statistical software and analyzed by herd using Proc Mixed with period of NutriTek feeding, test period, and month of test as independent variables. Cow was treated as a repeated observation. Least squares means for TRT (NutriTek or no NutriTek) were estimated for each herd. Clinical mastitis prevalence was tested by herd using Proc Logistic in SAS. Mastitis was the independent variable and TRT period and month of calving were the independent variables. Sixteen herds had a reduction (P < 0.05) in the prevalence of clinical mastitis cases and 8 herds had a tendency (P < 0.10) for a reduction in mean LS during the NutriTek feeding period. A purely random sampling should have resulted in only one-third of herds improving their linear scores and mastitis cases indicating that the herds fed NutriTek improved more than expected (odds ratio P < 0.05). Causative mechanisms for the above observations need to be explored to more fully describe this association.