Background. Current classification systems of neoplasms arising from renal parenchyma distinguish 5 categories of renal cell carcinoma (RCC), i.e. conventional RCC, papillary RCC, chromophobe RCC, collecting duct/medullary RCC and unclassified RCC. We present 13 cases of unusual and unclassified spindle and cuboidal renal cell carcinomas. Methods and Results. The studied group consisted of 13 patients (7 men and 6 women). They ranged in age from 22 to 65 years (mean 57.3). Generally, the tumours were well circumscribed and confined to the kidney, whitish to grey on section with a diameter 4,5–13 cm (mean 8.6 cm). One patient was investigated for loin pain and nocturia. Three patients had staghorn nephrolithiasis and vague sonographic findings in renal parenchyma. In one patient the renal tumour was found when examined on follow-up examination for prostatic adenocarcinoma.None of our patients was known to have elevated levels of parathyroid hormone due to hyperplasia, adenoma or carcinoma of the parathyroid gland. Clinical follow-up of the patients ranged from 9 months to 8 years (mean 2.3 years). Microscopically, the tumours were composed of two main populations of cells: flattened, spindle cells with sparse cytoplasm and small cuboidal cells with clear to light eosinophilic cytoplasm. Eight patients are currently well without signs of recurrence or metastasis, one had metastasis in the regional lymph node at the time of nephrectomy, one died of unrelated cause, and three were lost to follow-up.Conclusions. We present 13 cases of unclassified RCC. Our cases were histologically, immunohistochemically and ultrastructurally similar to the hitherto reported case reports of this variant of RCC. It is obvious, that that variant of RCC should be recognised as a new subtype of RCC.

        Key words: kidney, renal cell carcinoma, unclassified, spindle cell carcinoma.