OBJECTIVES: To estimate the cost of treatment of oncological pain with four strong opioids (methadone, morphine, oxycodone, fentanyl) in the Brazilian private healthcare system between January and December 2010. METHODS: A claims database of over 57 HMOs was analyzed to recover combined use of strong opioids and oncological treatments between January and December 2010. Records showed expenditure with medications, materials, hospitalization, and procedures and diagnostics.  RESULTS: Over the one-year study period, 293,918 patients made use of at least one of the four opioids. The total healthcare expenditure with these patients was R$ 3,243,890,302.91 (R$ 11,036.72/patient/year). Around 53% of these patients (157,104) made concomitant use of oncology treatments, representing around 74% of the total costs (R$ 2,424,503,643.76), with an average cost of R$ 15,432.48/patient/year. The remaining patients (136,814) had an average cost of R$ 5.989,06 per patient/year. Within the oncology patient population, the total healthcare expenditure with the four opioids alone was R$ 5,203,001.81. Fentanyl was the most commonly used opioid in about 66% of patients, followed by morphine (33%), methadone (1%) and oxicodone (0,8%). Around 17% of the oncology patient population made use of two or more opioids during the study period. CONCLUSIONS: Pain treatment of oncology patients is more costly for private payers in Brazil when compared with patients not receiving oncological treatment. Although 47% of patients were considered non-oncological, this is not certain as they could have received oncological treatment outside the study period or in a provider not covered by the database (e.g. public hospital).  With about 17% of oncological patients receiving two or more opioid treatments with the 12 month period suggests opioid rotation is common.