A 6-year-old boy presented with a unilateral renal tumour thought to be a Wilms' tumour. He had started a treatment but review of the histology and immunohistochemical studies proved the tumour to be a primary renal B-cell lymphoma. This case illustrates the importance of an open-minded approach in the histological examination of small round cell tumours in the kidney and immunohistochemical studies in such cases are strongly advocated.