Background <p>This study aimed to evaluate the financial impact, including medication-related costs and other costs, of conducting randomized controlled trials (RCTs) in the oncology department of a tertiary hospital.</p> Methods <p>This retrospective study included all clinical trials conducted in the oncology department from January 2018 to December 2023, regardless of the phase of clinical research or type of sponsor. The total avoided cost was calculated as the drug cost avoided minus other direct medical cost for activities performed in the hospital using data from the official publication of the autonomous community health authority.</p> Results <p>During the observation period, 197 clinical trials actively recruited 1,198 patients, 757 of whom completed their participation within the established timeframe and were dispensed trial medications. Drug cost avoidance totaled €25,899,148, with a median (interquartile range [IQR]) by trial of €61,034 (€5,280, €235,588). Both industry-sponsored and noncommercially sponsored studies were associated with drug cost avoidance with an absolute (relative) contribution to the total cost avoidance of €23,134,649 (89.3%) and €2,764,499 (10.7%), respectively. Other direct medical cost for activities performed in the hospital €3,274,099.80. The total cost avoidance was €22,625,048. There were large variations in cost avoidance depending on the type of tumor.</p> Conclusions <p>Although there are large variations across types of tumors, overall, clinical trials performed at the medical oncology department of a tertiary university hospital are associated with high cost avoidance and could contribute to reducing hospital expenditures for the management of cancer.</p> Trial registration <p>Not applicable.</p>