Zoledronic acid (ZOL) as a yearly infusion is effective in reducing fracture risk. An acute-phase reaction (APR), consisting of flu-like symptoms within 3 days after infusion, is commonly seen. The objective of this analysis was to investigate whether APR occurrence influences drug efficacy. This analysis uses data from the 3-year randomized clinical trial, Health Outcomes and Reduced Incidence with Zoledronic Acid Once Yearly-Pivotal Fracture Trial (HORIZON-PFT). APRs were identified as adverse events within 3 days of first infusion with higher frequency in ZOL than placebo. To compare mean 3-year change in bone mineral density (BMD) in ZOL versus placebo, among women with and without APR, t tests were used. Logistic regression was used to examine the relationship between APR occurrence and odds of incident morphometric vertebral fracture. Cox regression was used to determine the risk of nonvertebral and hip fractures for women with versus without APR. Logistic and Cox models were used to determine the risk of incident fracture in ZOL versus placebo for women with and without an APR. The analysis included 3862 women in the ZOL group and 3852 in placebo, with 42.4% in ZOL versus 11.8% in placebo experiencing an APR. The difference in BMD mean change for ZOL versus placebo was similar for women with and without an APR (all p interaction >0.10). Among ZOL women, those with APR had 51% lower vertebral fracture risk than those without (odds ratio [OR] = 0.49, p p = 0.10;​ RH = 0.70, p = 0.22, respectively). There was a greater treatment-related reduction in vertebral fracture risk among women with APR (OR = 0.19) than those without (OR = 0.38) (p interaction = 0.01). Our results suggest that women starting ZOL who experience an APR will have a larger reduction in vertebral fracture risk with ZOL.