Serum lactate dehydrogenase isoenzyme 1 catalytic concentration (S-LD-1) was measured at the time of orchiectomy in 104 patients with nonseminomatous testicular germ cell tumors (NSTGCT) clinical stage I who participated in a randomized study comparing surveillance after orchiectomy (group I) and radiotherapy (group II). For 68 patients, S-LD-1 was measured in a serum sample before or on the day of the orchiectomy. Twenty-seven patients (40%) had elevated S-LD-1; median 102 U/L (range 41-335). For the remaining 36 patients. S-LD-1 was measured in a serum sample after orchiectomy: 8 of these patients (22%) had elevated S-LD-1. S-LD-1 was normalized shortly after surgery in most patients with a preorchiectomy elevated S-LD-1. Fifteen of the 68 patients relapsed: 9 out of 27 with an elevated S-LD-1 and 6 out of 41 patients with normal S-LD-1 (p = 0.13, Fisher's exact test). In group 1, those with a preoperatively elevated S-LD-1 had a lower 8-years' relapse-free survival than those with a normal S-LD-1 (40% vs. 80%, p = 0.003, log-rank test). The role of S-LD-1 in the staging, prognostication and monitoring of patients with NSGCT clinical stage I should be further explored in a large, prospective study.