OBJECTIVES:: most health care providers from the Private Health Care System (PHS) in Brazil demand that oncologic treatments (OT) must comply with the best available literature and regulatory issues before being pre-authorized. Our goal was to assess which OT were denied approval and the economic impact they would have had for PHS in Brazil. METHODS:: anonymized demographic and clinical data were retrieved from Auditron® (Evidencias-Kantar Health proprietary database of claims covering 34 insurance companies) regarding OT requests during 2016. Reasons for denial were defined as scientific (when OT was unsupported by the literature) or regulatory (not compliant with PHS’ requests). We used SIMPRO values to calculate direct costs of denied OT. 1U$= R$ 3.3. RESULTS:: we retrieved 803 denied OT claims regarding 675 patients (346 males and 457 females) with mean age of 58 years. The most commonly denied claims were for patients with breast cancer (n=172; 21.4%); lung neoplasms (78; 9.71%); prostate (78; 9.71%), colon (75; 9.33%), pancreas (31; 3.8%) and ovary tumors (29; 3.61%). Denied requests came from all Brazilian regions: North (n=2), Central-West (202), Northeast (89), Southeast (375) and South (135). Direct costs for denied OT would have been R$ 18,561,796.64 (U$ 5,624,786.85). Most denials were for scientific reasons (710; 88.4%) such as maintenance of the same therapy after disease progression or lack of phase III trials supporting the treatment (costing R$ 17,334,658.46 or U$ 5,252,926.79). Regulatory issues, such as off-label use of medications and requests for drugs not included in the government lists were the reason for denying 93 (11.5%) claims costing R$ 1,227,138.18 or U$ 371,860.05. CONCLUSIONS:: Almost 90% of claims were denied by lack of scientific evidence. That suggests a gap in knowledge among oncologists regarding basic recommendations for the beginning and continuation of OT, which may be harmful for the patient and negatively impact the PHS' budget.