The prevalence of bilateral tumours was found to be 4% in a retrospective analysis of 531 consecutive patients with testicular germ cell tumours. Patients with bilateral tumours were younger when their first tumour was diagnosed than those who had only 1 testicular tumour. Bilateral undescended testis was more frequent in 21 patients with bilateral tumours than in the total population of patients examined (2 of 21 versus 10 of 452, i.e. 9.5% versus 2.2%). Histology and unilateral undescended testis were not helpful in identifying patients at risk of developing a second tumour. In a separate study, 76 consecutive patients with unilateral testicular cancer underwent contralateral testicular biopsy at the time of orchiectomy. Five patients had testicular intra-epithelial neoplasia (TIN, so-called carcinoma in situ) in the contralateral testis; 1 of these patients had a history of bilateral undescended testis and 4 had an increased level of serum follicle stimulating hormone post-operatively. None of the other 71 patients had developed a second tumour after a median observation time of 30 months. Testicular biopsy and a search for TIN appear to be a safe method for the early identification of the patients at risk of developing a contralateral testicular tumour.