Between 1979 and 1989, 135 patients with a non-seminomatous germ cell tumour of the tests were treated in the Val de Grâce Military Hospital in Paris. In twelve patients, histological examination of a residual mass after chemotherapy revealed the presence of mature teratoma. Although this tumour, considered to be benign, generally represents a favourable form of the initial tumour, it nevertheless has a potential for progression. The possibility of long-term recurrences, which are sometimes difficult to eradicate, justifies very careful long-term surveillance of these patients. The authors report three particularly demonstrative cases and present a review of the literature.