In a prospective study on the grading of superficial papillary neoplasms of the bladder a distinction was made between tumours showing only increased cellularity without appreciable cellular and nuclear deviation (grade 1), tumours showing slight cellular variation (grade 2a), and tumours showing clear cytological deviation and a tendency to lose normal polarity (grade 2b). Ninety-one patients with a superficial tumour were followed up for a mean of 24 months. Grade 2a tumours recurred later and in fewer patients than grade 2b tumours. Progression was seen in 4% of grade 2a tumours and in 33% of grade 2b carcinomas. Adapting our results to the WHO grading system, we suggest that all tumours in this study defined as grades 1 and 2a should be classified as low grade and tumours defined as grade 2b should be classified as intermediate grade.