The carcinogenic effects of microorganisms have been discovered in multiple cancer types. In urology, the development of squamous cell carcinoma of the bladder due to the parasitic infection with Schistosoma Mansoni is widely accepted. The oncogenic potential of biofilms has been studied in colorectal cancer and experimental studies have shown that bacteria such as Escherichia coli drive the development of colorectal cancer. Notably, Escherichia coli is responsible for 80% of all urinary tract infections. Recent findings suggest an altered urinary microbiome in patients with bladder cancer compared to healthy subjects. In this case series, we demonstrate our findings of biofilm formation in human bladder cancer tissue. Tissue samples from ten patients that underwent routine Transurethral Resection of Bladder Tumor (TURBT) were obtained from the Danish National Biobank. Pathological tissue was examined for presence of bacterial aggregates by Fluorescence in situ Hybridization. In two of ten patients, analysis showed abundant bacterial aggregation on the surface epithelium. Both positive cases had pT2 urothelial bladder cancer. Our findings suggest that biofilm occurs in urothelial cancer tissue indicating an association between biofilm formation and bladder cancer.