Immunohistochemically detectable p53 protein using a polyclonal antibody (CM-1) was studied in 42 carcinomas of which 11 were grade I, 22 grade II and nine grade III carcinomas. Additionally 14 urothelial dysplasias were studied. In 11 of these a diagnosis of transitional cell carcinoma was established before and in one after the dysplasia diagnosis. Twenty-one out of 42 (50%) cases of transitional cell carcinoma were positive for the p53 protein. Eleven out of 14 (78%) dysplasias and 10/12 (83%) related carcinomas were p53 positive. One out of 11 grade I (9%), 12/22 grade II (55%) and 8/9 grade III (89%) tumours showed positivity for p53. There were significantly more p53 positive cases in grade II-III tumours than in grade I tumours (P = 0.004). There were significantly more p53 positive cases in stage T2-T4 tumours than in stage T1 tumours (P = 0.035). In only one case among the 11 dysplastic lesions following the treatment of a carcinoma the dysplastic lesion was p53 negative while the preceding carcinoma was p53 positive. All dysplasias and 28 carcinomas were also immunostained for laminin and type IV collagen to evaluate the continuity of basement membranes (BMs). Clearly disrupted BMs were observed only in grade III carcinomas. These cases showed the most p53 immunopositivity. The results show a strong association of p53 staining between dysplasias and transitional cell carcinomas of the urinary bladder indicating that these lesions might share similar p53 changes. The correlation to grade, clinical stage and to disrupted BM suggests that p53 mutations may be associated with the evolution of aggressive growth characteristics in transitional cell carcinomas or, alternatively, that p53 positive tumours of a more aggressive type from the start. Whether p53 staining can be used as an adjunct in the assessment and follow-up of epithelial changes of patients treated for a p53 positive bladder carcinoma deserves to be studied.