Background Even though pharmacists have devoted considerable time to ensuring patient safety during the process of preparing and dispensing chemotherapy, only a few studies have evaluated their efforts. Objective To evaluate the clinical and economic impact of pharmacists’ interventions in a large volume chemotherapy preparation unit. Setting A 1600-bed tertiary hospital in Seoul, Korea. Method Pharmacist intervention records from May 2012 to April 2013 were retrospectively reviewed. The clinical significance of interventions was rated by one physician and one pharmacist. A cost-benefit analysis was conducted. The benefit from interventions was estimated through both cost avoidance based on the potential to avoid an adverse drug event (ADE) and cost savings related to reducing discarded products. Cost was estimated from the pharmacists’ salary corresponding to the time spent in reviewing chemotherapy prescriptions. Main outcome measure Acceptance rate, clinical significance, net cost-benefit, and cost-benefit ratio of pharmacist interventions. Results Among 39,649 cancer chemotherapy prescriptions in 6364 patients, 631 interventions were performed for 435 patients. The acceptance rate was 72.1 %. Most cases of declined interventions were related to dosage adjustment within the range of <10 % of the prescribed dosage. More than half of the interventions were considered as clinically more than “significant” (50.4 %). The cost-benefit analysis showed a clear cost benefit with a net cost-benefit of $116,493 and a cost-benefit ratio of 3.64:1. Conclusion Pharmacists’ interventions in a large volume ambulatory-based chemotherapy preparation unit provided a positive economic impact on health care budget and were effective in preventing clinically significant ADEs.