Contralateral biopsies taken from 1,188 patients with testicular germ-cell tumour were examined immunohistologically by staining for placental alkaline phosphatase. Testicular intraepithelial neoplasia (TIN; carcinoma in situ) was detected in 53 patients (4.5%; 95% confidence interval 3.3-5.7%). Testicular atrophy was found in 45.7% of cases with TIN and in 14.1% of cases without TIN (p < 0.01). History of undescended testis was present in 21.3% of cases with TIN and in only 9.3% in cases without (p < 0.05). Patients with contralateral TIN presented at an earlier age (31.0 vs. 33.7 years). Twenty cases with contralateral TIN had no specific risk marker. It is concluded that all patients with testicular germ cell tumour should be offered a contralateral biopsy.