Bladder tumours of patients o20 years have a low incidence of genetic aberrations typically found in tumours in older patients. In this study, we investigated oncogene mutations in patients with bladder cancer (BC) o20 years and compared them to older age groups. Interestingly, we observed a relatively high number of HRAS mutations in tumour from young patients. These mutations were also highly uncommon in BCs of older patients, ie, p.(Gly12Ser) and p.(Gly12Ala). Germline mutations in the HRAS gene, especially p.(Gly12Ser/Ala), cause Costello Syndrome (CS), a severe congenital disorder. Indeed, one of the patients had been diagnosed with CS. We hypothesized that some of the other patients might be mosaic for the HRAS mutation and therefore could express some of the clinical features of CS, like tumour predisposition. Hence, we isolated DNA from microdissected stroma and analysed it for HRAS mutations. In the CS patient and in patient X, the mutation was also highly expressed in normal stroma. We conclude that patient X is possibly mosaic for the HRAS mutation. These results suggest that mosaicism for oncogenic HRAS mutations may increase the risk for developing BC at a young age. (BC) is a disease of the elderly with a peak incidence in the sixth decade of life and only 1-2.4% of all cases present under the age of 40 years. BC in patients younger than 20 years is even more uncommon with reported incidence rates of only 0.1-0.4%. 1 Previous studies demonstrated that tumours of patients o20 years lack or have a much lower incidence of epigenetic and genetic aberrations typical of BC in elderly patients. 2 Together with mutations in FGFR3 and PIK3CA, mutations in the RAS genes are the most common mutations found in BC and up to 13% of all bladder tumours harbour a mutation in HRAS, KRAS or NRAS. 3 There is limited data available regarding these oncogenic mutations in BC of patients o20 years. Therefore, we aimed to investigate FGFR3, PIK3CA, and RAS mutations in BC of patients o20 years and compared the results with the mutation status of a control group consisting of patients with BC 420 years.