An increasing variety of dysplastic lesions of metanephric origin ('the nephroblastomatosis complex') have been recognized as precursors of Wilms' tumour. We describe here four cases of a hitherto unrecognized renal dysplasia associated with Wilms' tumour and having a panlobar distribution. It has many features in common with the intralobar form of nephroblastomatosis, but differs in extending through the full thickness of the renal substance from calyx to capsule. These cases are also characterized by an increased proportion of well-differentiated tissue elements and by the presence, in some cases, of anomalous metanephric differentiation in the pericalyceal tissues or renal capsule. The presence of Wilms' tissue arising deep in the renal substance should alert the pathologist to make a thorough search for associated dysplastic elements in the kidney.