Impaired immune function causes reproductive disease in many high-producing dairy cattle during the periparturient period, primarily due to a reduction in the number and function of neutrophils. When administered to periparturient dairy cattle, pegbovigrastim has been shown to dramatically increase circulating neutrophil counts, by stimulating their production and release from bone marrow. The objective of the research described in this thesis was to examine the effects of pegbovigrastim treatment on reproductive performance, and clinical disease and milk production in early lactation. The primary study was a large-scale, double-blind, randomized controlled trial, involving 1,607 cows in 6 commercial herds. Pegbovigrastim had no effect on retained placenta (RP), displaced abomasum, clinical mastitis, endometritis, or reproductive performance outcomes. However, cows with RP were less likely to develop metritis, heifers were more likely to have purulent vaginal discharge, and milk production was reduced in cows treated with pegbovigrastim, compared to controls.