Atypical spermatogonia are relatively frequent in the vicinity of testicular teratomas or seminomas. These voluminous cells are seminoma-like, showing broad and clear cytoplasm borders, big nuclei and peculiarly enlarged nucleoli. Atypical spermatogonia usually line the tubules and displace the remaining Sertoli cells towards the middle of the tubules. Recently, such atypical spermatogonia have been described in testicular biopsies performed for fertility disturbances. Two patients showing atypical spermatogonia developed, years later, malignant testicular tumors. Therefore, were checked all of our histological specimens from testicular biopsies over the period 1950-1976 for atypical spermatogonia. They originated from 1935 adult patients in whom biopsy had been performed, in general bilaterally, for fertility disturbances. In fact, atypical spermatogonia were found in the specimens of 9 patients, that is, 0.55%. Five of these patients developed malignant testicular tumours within periods ranging from months to six years, viz. three seminomas, one teratoma and one combined tumor, i.e. a teratoma and seminoma. The remaining four patients with atypical spermatogonia have shown no sign of tumor to date. As the results of our investigation show, atypical spermatogonia in testicular biopsies should not be taken lightly. We therefore strongly advise checks at short intervals on such patients in view of the high risk of their developing malignant testicular tumors.