OBJECTIVES: This analysis was done to assess if co-morbidities influence the public payer (PP) cost of dialysis patients in Belgium. METHODS: The data from a cost study (retrospective chart review of 130 Belgian patients undergoing dialysis in 2006) was analyzed a posteriori. Baseline medical characteristics were used to compute the Charlson co-morbidity score (CCMS). Costs included: dialysis procedure and medical management (i.e., hospitalizations, outpatient visits and procedures, laboratory and imaging tests, and transport). Multivariate analyses were performed with the logarithmic transformation of costs as the dependent variable and CCMS, dialysis modality (hemodialysis: HD or peritoneal dialysis: PD) and gender as the independent variables. The regression model was weighted by number of patient months in the study. CCMS was categorized as low (<4), moderate (4-5), high (6-7) and very high (>=8). RESULTS: All 3 variables had a significant impact on costs. Total costs to the PP were 16% higher for HD than for PD patients (p=0.0039) and were 13% higher in women than in men (p=0.0207). The costs in patients with a very high CCMS were 21% higher than those with a low or moderate score (p=0.0072 and p=0.0094 respectively) and 10.7% higher than those with a high score, but this latter difference did not reach statistical significance (p=0.1160). The differences were larger when excluding the cost of dialysis procedure and considering medical management only, but only reached statistical significance or patients having a very high CCMS score vs. low or moderate CCMS (p=0.0036 and 0.0056 respectively).  CONCLUSIONS: This analysis showed that patient co-morbidities have a significant impact on medical management and total costs of dialysis patients. It is therefore important to take this into consideration when studying the costs of dialysis patients, especially if a total cost approach (i.e., procedure plus medical management) is taken.