The authors test a hypothesis retrospectively in two cohorts of patients with non-muscle invasive bladder cancer (NMIBC), that endogenous bacteria in the bladder might exert antitumour effects, similar to live mycobacteria (BCG), through local immune-related or other mechanisms, on NMIBC treated by transurethral resection (TUR) with or without concomitant BCG therapy. Asymptomatic bacteriuria (ABU) is common in elderly patients, and it is shown that cystoscopy to detect and treat recurrences is safe in ABU (>10⁴ or >10⁵cfu/ml), and do not require antibiotics. The first cohort, Ta low grade (TaLG) patients at high-risk for recurrence, (≥1 recurrence per year), underwent three to six monthly cystoscopies, with urine analyses before each cystoscopy. At three years, 44% developed recurrence (median: 14.5 months). Compared with uninfected patients, the absolute recurrence (28% vs. 49%; P=0.001) and time to recurrence (P=0.002) favoured those with ABU. The second cohort had restaging TUR to resect high-grade NMIBC, received at least one induction course of BCG therapy, and was followed for two years. Some had a second induction course, but no maintenance BCG. Before starting BCG treatment or cystoscopy, urine cultures were done. Follow-up included serial urine cultures, cystoscopy, repeat TUR as necessary every three months for the first year, then every six months in the second year. The overall response rates to BCG were better in infected patients (88% vs. 81%), but P=0.09. Absolute rate of recurrence (18% vs. 28%, P=0.03) and the time to recurrence (P=0.02) favoured those with ABU. No significant difference was seen in progression-free survival. In both cohorts, Enterococcus and E.coli were the predominant organisms and more common in women. Those with chronic ABU had fewer recurrences, fewer numbers of recurrent tumours, and longer tumour-free survival times than similarly staged uninfected patients, the mechanism of which is unknown. This bacterial priming of complex immune responses acting to retard tumour growth is worth further study.