Random urothelium biopsies were taken at initial endoscopic surgery from 1001 patients with primary superficial bladder cancer. The clinical course of all the patients was assessed prospectively. Actuarial risks of recurrence and disease progression were determined for prognostic characteristics and comparisons were made using log-rank tests. The independent prognostic significance of concomitant intra-aurothelial dysplastic changes was examined with Cox's regression analyses. The 3-year risk of recurrence in patients with dysplasia and carcinoma in situ (CIS) in macroscopically normal-looking urothelium was only slightly higher than the risk in patients without dysplastic changes (56, 58 and 51%, respectively; P = 0.25). Concomitant dysplasia or CIS significantly increased the 3-year risk of disease progression (17 and 31%, respectively, versus 7%; P < 0.001). After adjustment for the effects of age, tumour stage, grade, size and multicentricity, the result of random biopsies had no prognostic significance regarding the risk of recurrence, but the detection of dysplasia or CIS increased the risk of progression by approximately 80%. This result suggests that random urothelium biopsies may be useful as an additional guide in defining therapy in primary superficial bladder cancer.