229 patients with Grade 1–2 tumours (WHO), all category Ta or T1 (UICC) and surgically treated, were followed clinically and by flowcytofluorometric DNA-analysis (FCM). The tumours were characterised by their DNA profile. 175 cases were found to be diploid and fiftyfour cases showed aneuploidy. The mean follow-up time with continous FCM analysis was 2.6 years. During this period 19 patients showed tumour progression and 11 of these patients died. No progressive cases were found among 175 patients with repeatedly diploid DNA patterns. Thus tumour progression was exclusively linked to an aneuploid DNA pattern. In these case the degree of ploidy determined the frequency of progression: while 50% of the cases with triploid — hypotetraploid DNA pattern showed progression, only 10% of tumours with a tetraploid amount of DNA were found to be progressive. The degree of ploidy in 33 cases with recurrent aneuploid tumours was in general found to be constant. A fairly high degree of consistency was also found in the number of cells in S-phase, expressing proliferative properties. This indicates that superficial bladder tumours can be well characterised by their DNA profiles, that is the degree of ploidy and the proliferation pattern.