Expression of a newly described inhibitor of tumour metastasis, KAI1, was examined in bladder cancer progression and compared with the expression of p53 and pRb, which are markers of advanced disease. KAI1 mRNA (by in situ hybridization) and protein levels (by immunohistochemistry) were examined in 135 paraf®n-embedded bladder tissue sections. Signi®cant decreases in KAI1 mRNA and protein levels were detected between normal and tumour tissue ( p<0.001 and p=0.026, respectively), and between non-invasive and invasive tumours ( p=0.046 and p<0.001, respectively). Loss of KAI1 protein expression was accompanied by a shift in staining pattern from a uniform distribution to a weaker, membranous or heterogeneous pattern. Normal tissue and low-grade tumours showed little p53 protein staining. High level staining (indicative of mutant p53) was associated with increased grade in non-invasive tumours ( p=0.031) but was not signi®cantly higher in invasive tumours. Whilst p53 protein staining increased with malignant progression and KAI1 mRNA expression decreased, there was no signi®cant correlation between the two patterns ( p=0.33, adjusted for group, p=0.18) or when only cancer samples were analysed ( p=0.065, adjusted for group, p=0.26), even when taking into account overexpression of MDM-2 protein as a pathway for inactivation of p53. There was no correlation between loss of KAI1 mRNA expression and gain of abnormal pRb staining ( p=0.30, or adjusted for tumour samples only, p=0.59). These results suggest that loss of KAI1 expression is associated with invasive bladder cancer, but is not related to mutation of p53 or to loss of normal pRb expression.