A light microscopical study on a total of 812 consecutive testicular biopsies from 555 infertile men revealed intratubular changes in germ cells compatible with a carcinoma in situ pattern in six oligospermic patients (I.I%); the changes were found in both testes in two of these men. Four of the six patients developed an invasive germ cell tumour within follow-up period of 1.3 to 4.5 years. The results confirm the malignant nature of these intratubular atypical germ cells. It is concluded that testicular biopsy may be useful for early detection and cure of germ cell carcinoma in patients at risk, i.e. patients with cryptorchidism, infertile men or patients with previous cancer of one testis.