Development of second testicular tumours, i.e. bilateral testicular cancer, is influenced by systemic chemotherapy for the first tumour. The prevalence of bilateral testicular cancer was studied in patients with initial stage I disease, in which no systemic treatment was given after orchidectomy. All stage I testicular cancer patients entered a surveillance study with an intensive follow-up since 1982. We hypothesised that after 1982, bilateral testicular cancer was diagnosed at an earlier stage of disease. The prevalence of bilateral testicular cancer was 4.7% (8/170) in stage I patients treated between 1967 and 1981, and 2.9% (8/275) in stage I patients treated between 1982 and 1997 (P > 0.5 chi 2-test). In the period 1967-1981, 6 patients had stage I second tumours and 2 patients had stage III second tumours. The former 6 patients are alive with no evidence of disease and the 2 patients with metastatic tumours died of disease or treatment. In the period 1982-1977, all 8 patients had stage I second tumours and all are alive with no evidence of disease. The overall prevalence of bilateral testicular cancer in stage I patients was 3.6% and has slightly decreased over the past three decades. Intensive follow-up, improvement of radiodiagnostic computed tomography techniques, availability of serum tumour markers, and patient education have resulted in earlier diagnosis and lower stage of contralateral testicular tumours, contributing to improved prognosis.