OBJECTIVES: To examine the effects of comorbidities on the hospital readmission rate in renal failure (RF) patients on dialysis. METHODS: We used 2005-2007 U.S. MarketScan claims database to identify RF patients under 64 years old. RF dialysis patients were identified using ICD-9 and CPT codes. RF patients had to be continuously eligible for at least 6 months after the initial dialysis diagnosis date (index dates). Comorbidity scores were measured by the Charlson Comorbidity Index (CCI). We identified readmission rates to the hospital within 15 days after the index date. ANOVA tests and logistic regression were performed to compare outcomes by CCI. RESULTS: A total of 6117 patients were diagnosed with RF.  Among those who initiated dialysis treatment, 13.9% and 86.1% were treated with peritoneal dialysis and hemodialysis, respectively. Over half of all patients were male (55.2%) and the average age was 52.4 years. The most frequent comorbid conditions were hypertension (69.1%), diabetes (34.5%), congestive heart failure (CHF) (26.3%), and anemia (26.4 %). Most patients had low (78%, score