Cellular DNA content was determined by flow cytometry on routinely processed paraffin sections of 61 primary and untreated transitional cell carcinomas of the urinary bladder, and correlated with tumour grade and stage and clinical follow up. All 16 (25%) grade 1 carcinomas were diploid and all 11 (20%) grade 3 tumours were aneuploid. The 34 (55%) grade 2 carcinomas comprised 13 (40%) diploid and 21 (60%) aneuploid cases. Among the 37 superficial carcinomas (stage Ta and T1), 25 (65%) were diploid; 20 (85%) of the 24 advanced tumours (stage T2 to T4) had aneuploid tracings. Ploidy was a significant prognostic indicator (p: 0.006) of five year survival. The initial presence of aneuploidy in superficial bladder carcinoma (stage Ta and T1) is a strong argument for more aggressive treatment than is customary.