The activities of serum lactate dehydrogenase (S-LDH) and S-LDH isoenzymes were determined in 252 patients with a history of testicular germ cell tumors (TGCT). Fifteen of 37 patients with TGCT lesions and seven of 215 without had raised levels of S-LDH (above 8.0 mukat/l (480 U/l)). Of the patients with TGCT lesions, four had only raised S-LDH-1 levels, one only raised S-LDH-2 (and normal S-LDH), two only raised S-LDH-3 (one with normal S-LDH), and 10 had five combinations of raised levels of S-LDH isoenzymes with a predominance of S-LDH-1. S-LDH and S-LDH-1 correlated significantly with the total tumor volume in the patients with TGCT lesions, especially pronounced in those with lesions from seminoma. Of 34 patients with TGCT metastases, 13 with raised S-LDH levels lived significantly shorter lengths of time than 21 with normal S-LDH. Similarly, 11 with raised S-LDH-1 (above 3.0 mukat/l (180 U/l) lived significantly shorter times than 23 with normal S-LDH-1. S-LDH is a valuable tumor marker in patients with TGCT, especially in those with seminoma. Routine determination of S-LDH isoenzymes in addition to S-LDH in patients with TGCT is not recommended. In patients with a history of TGCT and an unexplained elevation of S-LDH levels, a raised S-LDH-1 level indicates the presence of TGCT lesions.