The main haematological parameters, particularly haemoglobin concentration and the numbers of leukocytes and platelets, were studied in 146 kidney tumour patients. Only 119 of the subjects studied later underwent nephrectomy. In 66 patients the tumours were limited to within the renal capsule (group I) while in 39 other subjects the tumours had gone beyond this limit and had invaded the perirenal tissue (group II). 27 patients had metastasis in one or more sites (group III). In the remaining 14 subjects (group IV) without proven metastasis, the surgical staging was not available since because of cachexia they were not undergone nephrectomy. More frequent signs were: anaemia (Hb less than 12 g/dl), mainly normochromic, found in a high percentage of cases (41% in group I, 59% in group II, 74% in group III, 79% in group IV); leukocytosis (G.B. greater than 8000/mm3: 38% in group I, 51% in group II, 37% in group III, 50% in group IV). Mild thrombocytosis (P = 350-500 X 10(3)/mm3) was found with notable frequency mainly in groups II and III (20% and 18% respectively). On the other hand leukopenia (G.B. less than 3500 per mm3) and thrombocytopenia (P less than 140 X 10(3) per mm3) were rather rare. The results of our study are also discussed in relation to the main findings in the literature.