The clinical significance of serum lactate dehydrogenase (LDH) and serum human choriogonadotrophin (HCG) as tumour markers was assessed in 105 patients with pure seminoma from whom 981 blood samples were analysed. The specificity of elevated HCG and LDH was 100 and 93% respectively. The comparable sensitivity was 32 and 47%. Serum LDH could not discriminate between patients with clinical stage I seminoma, prior to orchiectomy, and those with benign testicular lesions. In patients with advanced metastatic seminoma subjected to orchiectomy, serum LDH was increased in 82%, but elevated HCG was found in only 40%. After cisplatin-based chemotherapy, falsely elevated LDH was observed in 7 of 37 tumour-free patients, but HCG was normal in all patients with no evidence of disease. Six patients with residual tumour after chemotherapy had normal LDH and 4 of them had elevated HCG; 70% of the relapses in seminoma patients were associated with increased LDH (64%) and/or HCG (48%). In seminoma patients with comparable disease extension, elevated HCG seemed to be correlated with a high risk of relapse. Patients with normal pre-treatment LDH had a lower relapse-free survival rate than patients with elevated LDH. HCG is a highly specific tumour marker in seminoma with a rather low sensitivity. HCG is particularly useful for the primary diagnosis in patients with testicular lesions and during monitoring of chemotherapy in seminoma patients. LDH is less specific than HCG. Both markers should be analysed during follow-up of seminoma patients, since 70% of relapses are associated with an increase in one or both markers. Elevated pre-treatment HCG, but not elevated LDH, seems to indicate an increased risk of relapse in patients with seminoma.