The risk of smoking-related secondary cancers developing in bladder cancer patients was studied. The study population consisted of 10,014 bladder cancer patients reported to the Finnish Cancer Registry between 1953 and 1989. The risk of contracting a new primary cancer was estimated as a standardized incidence ratio, defined as the ratio of the observed and expected numbers of cases. Of 660 secondary cancers (6.6%) observed (standardized incidence ratio 0.96) 44% were considered to be smoking-related. Lung cancer was the most common secondary cancer (30% overall), and it occurred significantly more often than expected (standardized incidence ratio 1.31, 95% confidence interval 1.13 to 1.50). Also, larynx cancer among men (standardized incidence ratio 1.67, 95% confidence interval 0.95 to 2.79) and kidney cancer among women (standardized incidence ratio 3.55, 95% confidence interval 1.84 to 6.20) were found more often than expected. These excess risks were observed up to 20 years after diagnosis of bladder cancer. Therefore, bladder cancer patients experience an excess risk of smoking-related new tumors, which must be acknowledged during the initial evaluation and regular followup of such patients.