Bilateral disease occurs in 6% of patients with Wilms' tumour. Bilateral renal involvement is present in 25%–50% of children with B-cell non-Hodgkin's lymphoma. Paediatric oncologists therefore encounter bilateral renal disease relatively frequently.A patient with bilateral Wilms' tumour is presented, in whom at least 16 synchronous tumours in the kidneys were treated successfully by primary chemotherapy and ‘nephron- sparing’ surgery, without renal radiotherapy. We believe the successful treatment without radiotherapy will allow greater potential for normal growth in the future.The case history of a patient with renal failure due to the infiltration of both kidneys by lymphoma and who was treated successfully by chemotherapy, is also presented. One kidney has completely returned to normal function and growth; the other has completely failed and almost disappeared, demonstrating complete reversibility of the damage up to some critical point, beyond which, failure and atrophy result.These two case histories demonstrate extreme examples of clinical dilemmas involving bilateral renal tumours and allow discussion of modern management aimed at preserving renal function.