Use of antimicrobials for treatment of animals is under scrutiny and knowledge about frequency of antimicrobial treatment in dairy cows is needed. The objective of this study was to determine associations of farm size and milk production with antibiotic usage (AMU) on larger WI dairy farms. Eligible farms contained >250 lactating cows and met specified criteria for recording diseases and treatments in DairyComp 305. Enrolled farms (n = 38) submitted data before a farm visit during which an interview was performed to validate disease diagnosis, definitions and recording accuracy. Farms contained 52,563 cows and an average of 1251 (±332 SE) lactating cows per herd. Herds were categorized as “medium” (<1,000 total cows (n = 22) or “large” (≥1,000 (n = 16) and “low” (≤12,700 kg/cow (n = 22) or “high” (>12,700 (n = 16) producers. One-way ANOVA was used to identify associations of categories (production or herd size) with selected disease frequencies and AMU rate. Descriptive statistic was used to calculate the frequency and rate of disease. Frequencies (events/100 cows/yr) of diseases were 34.2 (±3.9), 27.8 (±3.2), 79.7 (±1.2), 3.4 (±0.644), 2.3 (±0.373), 10.5 (±1.7), 3.7 (±0.8) for all mastitis, clinical mastitis, dry cow treatment, respiratory disease, displaced abomasum, metritis and retained placenta, respectively. Frequencies of antibiotic treatment (treatments/100cases/yr) of mastitis, clinical mastitis, dry cow treatment, respiratory disease, displaced abomasum, metritis and retained placenta were 72.7 (±5.2), 69.1 (±5.5), 98.5 (±0.8), 84.5 (±4.5), 74.0 (±5.4), 78.9 (±5.5), 48.2 (±7.2) respectively. No associations between milk production or farm size were identified for mastitis (including clinical cases), displaced abomasum or dry cow treatment. Respiratory disease treatment rate was associated with herd size (0.78, ± 0.05 (medium), (0.93, ± 0.6 (large); P = 0.105) and production (0.77 ± 0.05 (low), (0.94 ± 0.06 (high) P = 0.105) as well as metritis treatment rate (0.69 ± 0.06 (low), (0.92 ± 0.08 (high); P = 0.03) and retained placenta treatment rate (0.33 ± 0.08 (low), (0.68 ± 0.1 (high) P = 0.01) were associated with production. Diseases and AMU varied among farms and for some diseases, production and herd size category were positively associated with AMU.