OBJECTIVES: To estimate costs associated with adverse events (AEs) in patients aged ≥65 years receiving treatment for mRCC. METHODS: Retrospective study using the linked SEER (Surveillance, Epidemiology and End Results) Medicare database.  Study subjects consisted of all persons in the linked SEER-Medicare database, aged ≥65 years, with evidence of newly diagnosed mRCC between January 1, 2007 and December 31, 2007.  AEs of interest comprised abdominal pain, back pain, diarrhea, dyspnea, extremity pain, fatigue/asthenia, hand-foot syndrome, hypertension, lymphopenia, nausea/vomiting, neutropenia, and proteinuria.  Patients receiving care for these AEs were identified using ICD-9-CM diagnosis codes on Medicare claims.  Costs were examined over a 30-day period, beginning with date of first mention of each AE; non-evented patients were assigned a “shadow” index date for comparison purposes.  We estimated total costs over 30 days following the index date for patients with and without AEs, on both an unadjusted basis and following adjustment for differences in baseline characteristics using a generalized linear model (GLM).  RESULTS: Sixty percent of patients receiving treatment for mRCC had healthcare encounters for one or more AEs.  Those occurring 20% or greater frequency included severe abdominal pain, dyspnea, and fatigue/asthenia; 10-20% of patients had encounters for back pain, extremity pain, and nausea/vomiting.  Mean (SD) total costs of care during the 30-day, post-index period were substantially higher among patients with AEs ($13,944 [$14,529] vs. $1,878 [$5,264] for those without these events).  Adjusting for differences in baseline characteristics, the estimated cost difference (95% CI) was $11,454 ($7,909 - $16,319).  CONCLUSIONS: Costs of AEs associated with treatment of mRCC are high in patients aged ≥65 years.  Efforts to prevent and/or better manage these events may reduce health care costs.