OBJECTIVE To estimate the risk of a second primary tumour (SPT) of the bladder in a cohort of childhood cancer survivors, investigate factors associated with a bladder SPT developing, and compare the risk observed with that expected from the general population. METHODS The analysis included 17981 individuals diagnosed with childhood cancer, between 1940 and 1991 in Britain, and surviving for ≥5 years. Ascertainment of a bladder SPT was primarily through the National Health Service Central Registers (NHSCR). RESULTS From the NHSCR, 17 bladder SPTs were ascertained; this corresponded to four times (95% confidence interval 2.5-6.4) the expected number of bladder tumours. Standardized incidence ratios (SIRs) varied significantly (P < 0.05) by first primary tumour (FPT) type, follow-up period, attained age and chemotherapy. The highest SIRs were in those: with heritable retinoblastoma (31.4); treated with chemotherapy (12.0); 0-9 years of follow-up (10.8); and aged 0-19 years (9.3). The absolute excess risk (AER) for a bladder SPT was 3.7 cases/100000 survivors per year. The AER varied significantly by FPT type, follow-up period, attained age and gender. The highest AERs were in those: diagnosed with heritable retinoblastoma (34.0); 20-29 years of follow-up (14.2); aged 40-49 years (13.0); and male (5.8). Using multivariable Cox regression, FPT and chemotherapy were significantly associated with the risk of a bladder SPT developing. By the age of 55 years, 0.4% of survivors developed a bladder SPT. CONCLUSIONS Although the absolute risk of a bladder tumour within childhood cancer survivors was low, the risk was four times that expected from the general population. Specific groups, e.g. survivors of heritable retinoblastoma and those treated with chemotherapy, were at the highest risk.