BMI-1 regulates cell proliferation and differentiation, is involved in stem cell maintenance and can act as an oncogene. We investigated BMI-1 expression in healthy normal kidney and in 77 renal tumours by immunohistochemistry, and correlated it with tumour differentiation. BMI-1 could regularly be demonstrated in distal tubules and in Bowman's capsule, whereas it was mostly lacking in proximal tubules, indicating that it may rather be a differentiation marker of different renal cell populations than a stem cell marker. In contrast to previous studies demonstrating a correlation between BMI-1 expression and malignancy, we showed that its expression was inversely correlated with the differentiation grade of clear cell carcinoma. Furthermore, despite their different biologies, BMI-1 was strongly expressed in both papillary carcinomas and oncocytomas. Thus, in renal clear cell carcinomas BMI-1 is rather a differentiation marker lost in carcinomas with high malignancy than an oncogene involved in tumour progression.