The authors report a patient affected with mental retardation, dysarthria, bilateral testicular hypoplasia and extensive ulcers of the lower limbs. Clinical study and laboratory tests revealed 48,XXYY syndrome. The authors confirm the importance of differential diagnosis from Klinefelter syndrome, illustrating the parameters and the pathology of both syndromes. They discuss the hypotheses concerning the pathogenesis of the ulcerations, and stress the importance of clinical and genetic characterization, leading to a differentiated prognosis of social capacity and prospect of working.