Serum lactate dehydrogenase isoenzyme I catalytic concentration (S-LD-1) was measured in patients with testicular seminoma clinical stage I followed with surveillance after orchiectomy. The serum samples were obtained before orchiectomy in 110 patients (group A) and soon after orchiectomy in 55 patients (group B). In group A, 60 patients (55%) had elevated S-LD-1 and 10 patients (9%) had elevated serum human chorionic gonadotropin concentrations (S-hCG). In group B, median S-LD-1 was lower than that of group A and decreased with increasing time after orchiectomv (p = 0.001, Jonckheere-Terpstra test, one-sided). After a median follow-up of 5.1 years, 23 patients (21%) in group A had relapses. The patients with elevated S-LD-1 and those with normal S-LD-1 had a similar relapse-free survival (p = 0.79, log-rank test). Thus patients with seminoma stage I had elevated S-LD-1 more often than elevated S-hCG but an elevation in S-LD-1 did not predict a relapse during follow-up with surveillance. Further studies are required to elucidate the value of S-LD-1 in monitoring the surveillance of patients with seminoma stage I.