Background. The incidence of renal replacement therapy (RRT) among patients aged >= 65 years is much higher in Flanders, the Dutch speaking region of Belgium, than in the Netherlands. We studied whether differences in referral policy to nephrologists by primary care physicians (PCPs) and specialists between Flanders and the Netherlands may play a role. Methods. A vignette study was performed among 329 PCPs and 96 specialists in Flanders and compared to the vignette study that was conducted among 209 PCPs and 162 specialists in the Netherlands. Physicians were offered six vignettes concerning case reports of patients with chronic kidney disease and varying co-morbidities or social circumstances. Each vignette was presented for a 65- and an 80-year-old patient. Physicians were asked about the likelihood of referral of the patients in the given circumstances. Univariate and logistic regression analyses were performed to identify whether country affected the likelihood of referral. Results. Univariate analyses showed that the percentage of PCPs who would probably or definitely refer a 65- or an 80-year-old patient with less severe co-morbidity was significantly (P