The clinical features of 19 patients with malignant infiltration of the retroperitoneal tissues are described. These patients usually presented with unexplained uraemia and nonspecific symptoms; only a few had other evidence of malignancy. The diagnosis was resolved only by histological examination of multiple biopsy specimens. Since the prognosis is usually very poor it is essential to distinguish this condition from non-malignant causes of retroperitoneal fibrosis.