According to pretreatment values of serum lactate dehydrogenase (LDH), 113 consecutive patients with non-Hodgkin's lymphoma were divided into three levels: level 1 (within normal range) with LDH less than 250 U/l; level 2 (moderately increased) with LDH between 250 and 500 U/l; level 3 (highly increased) with LDH more than 500 U/l. LDH was elevated in 46 of 113 patients (41%). Normal values of LDH were associated with a better response to therapy and a longer survival, independent of histological type and clinical stage, with one exception; in stage IV patients conclusions could not be drawn concerning the response to therapy (complete remission occurred only in 8 of 44). Even though level 2 patients behaved slightly better than level 3 patients, no statistical difference has been observed between the two levels. Accordingly, serum LDH can be considered a useful predictor of response to therapy and of survival in non-Hodgkin's lymphoma.