The authors report a case of renal cell carcinoma in a horseshoe kidney and discuss diagnostic and therapeutical aspects arising from this combination. Renal carcinoid tumors and nephroblastoma seems to be more frequent in this malformation. Diagnosis of horseshoe kidney may be difficult and angiography examination is very useful to confirm renal anomaly, tumor situation, and to plan the surgical approach because vascularization vary case to case. Heminephrectomy adapted to neoplasic localisation remains the essential treatment.