OBJECTIVE We performed contra-lateral testicular biopsies in 55 testicular tumor patients when high orchiectomy was performed. In these cases, two cases developed invasive testicular tumor later although the biopsies had not revealed testicular CIS. Then we re-examined the sensitivity of biopsies and judged if our results are contradictory against Skakkebaek's theory. METHODS The paraffin blocks of two cases who later developed testicular tumor were sliced again and re-examined by H/E staining and immunostaining with PLAP antibody (clone No. 8A9). The other 53 H/E samples were re-examined and the result of the contra-lateral testis was re-searched in the case that CIS was detected in the specimen. RESULTS CIS was detected in one of the two cases who later developed contra-lateral testicular tumor and another case among the other 53 cases. We could not reveal the result of the testis of case No. 3 because of the patient's disappearance. CIS existed 3.6% (2/55) and two cases were found to have been false negative. CONCLUSIONS It is important for both urologists and pathologists to know well about testicular CIS and to perform biopsy according to Skakkebaek's guidance for raising the sensitivity to detect testicular CIS.