Trends in cancer occurrence and survival may reflect changing risks and prognosis, respectively, but may also be caused by changes in detection, classification and registration. Changed classification of low-stage papillary carcinomas may have a material effect on observed trends in the occurrence of bladder cancer. We studied the effect of the implementation of the WHO grading system and the third edition of the TNM staging system on bladder cancer incidence in the south-eastern part of the Netherlands. Data on superficial and invasive bladder cancer incidence between 1975 and 1989 were derived from the population-based Eindhoven cancer registry. Data on survival of patients with stages I-IV bladder cancer were derived from the municipal population registers. Age-adjusted bladder cancer incidence per 100,000 person-years rose from 25.9 to 40.7 in males and from 3.1 to 8.5 in females. This increasing trend was caused almost entirely by non-invasive pTa papillary carcinoma. A considerable shift was observed towards lower disease stages, which was less evident within the group of invasive tumours. The relative 5-year survival of patients with stages I-IV invasive bladder cancer was 59% in 1975-1977 and 70% in 1984-1986. After stratification by stage, however, no striking improvement was observed in the prognosis. We conclude that the increasing trend of bladder cancer occurrence in the Netherlands since 1975 has largely been caused by changed classification systems and reporting procedures for pTa tumours (formerly classified as papillomas).