The clinical significance of neuron-specific enolase (NSE) as a tumour marker was evaluated in 54 patients with seminoma. Before orchiectomy NSE was elevated in six out of 21 patients with stage I seminoma and 11 out of 16 patients with metastases. After orchiectomy NSE normalised in all evaluated stage I cases, but was still elevated in six out of 12 patients with metastatic disease. NSE monitored the effect of cisplatin-based chemotherapy in patients with metastases. In some patients, increased serum NSE was found together with raised levels of human choriogonadotropin (HCG) and lactate dehydrogenase (LDH), while in others only NSE was elevated. No false positive NSE values were observed. NSE seems to be a clinically worthwhile serum tumour marker for monitoring seminoma patients, with a sensitivity and specificity of the same order as HCG.