The value of serum deoxythymidine kinase (TK) for the staging and evaluation of disease activity of non-Hodgkin lymphoma (NHL) as compared with serum beta 2-microglobulin, serum lactate dehydrogenase, blood sedimentation rate, blood hemoglobin, white blood cell count, lymphocyte count and platelet count was investigated in 101 patients. In addition, the performance status was determined by the Karnofsky index. Patients with chronic lymphocytic leukemia (CLL; n = 43) and immunocytoma (IC; n = 19) were staged according to the Binet classification, and the other low (n = 28) and high grade NHL (n = 8) according to the Ann Arbor classification. The analysis of all CLL and IC patients revealed that TK values correlated better with Binet stages (p = 0.01; n = 58) than blood sedimentation rate (p = 0.05, n = 12), lactate dehydrogenase (p = 0.08; n = 50), beta 2-microglobulin (p = 0.29; n = 28), lymphocyte count (p = 0.70; n = 57), white blood cell count (p = 0.69, n = 59) and the Karnofsky index (p = 0.16, n = 50). Mean TK levels of these patients were for Binet stage A 6.2 +/- 0.8 U/l (mean +/- S.E.M., range 2.3-18.0), stage B 13.3 +/- 6.5 U/l (3.8-38.8) and stage C 19.6 +/- 4.4 U/l (1.9-79.0), and for 22 healthy controls 3.8 +/- 0.2 U/l (2.2-6.0). Patients with multiple courses of chemotherapy (n = 32) previous to the study had significantly (p = 0.01) higher TK levels (16.4 +/- 3.7 U/l; 2.3-79.0) than those with only up to one course (n = 66; TK: 8.6 +/- 1.4 U/l; 1.5-66.3). The follow-up of 16 patients with low grade NHL showed that serum TK levels paralleled well the clinical response. The results indicate that TK might be a worthful parameter to estimate progression and response to therapy of NHL.