Aerobic glycolysis increases in tumor cells and pyruvate kinase (PK) is one of the key enzymes involved; PK exists in different isoforms in various tissues. Tumor M2-PK (TM2-PK) is one of these isoforms and its expression has been observed in various tumor cells, including lymphocytes, and in lymphoproliferative disorders. The present study aimed to compare plasma levels of TM2-PK and serum levels of two established markers of various lymphoproliferative disorders-lactate dehydrogenase (LDH) and beta-2 microglobulin, and to evaluate the role of TM2-PK in drug monitorization and disease activity in mycosis fungoides (MF) patients. The study included 27 MF patients and 46 healthy controls. Among the MF patients, 18 were stage IA, 6 were stage IB, 1 was stage IIA, and 2 were stage III. Plasma TM2-PK, and serum LDH and beta-2 microglobulin levels in the patients and controls were measured using the ELISA technique, a kinetic method, and a chemiluminescent assay, respectively. Measurements were repeated in the patient group posttreatment. Median levels of TM2-PK, LDH, and beta-2 microglobulin level in the MF patients were 22 U mL(-1), 375 U L-1, and 1,831 ng mL(-1), respectively. TM2-PK and beta-2 microglobulin levels did not significantly differ between the MF patients and controls; however, LDH levels were significantly higher in the MF patients. TM2-PK levels in 17 of the MF patients that were in remission did not significantly differ from their pre-therapy levels. Based on a cut-off point of 17.5 U mL(-1), the sensitivity and specificity of TM2-PK for diagnosing MF were 55.6 and 60.9%, respectively. beta-2 microglobulin was the most sensitive marker for diagnosing MF (63%), while LDH was the most specific marker. Furthermore, the sensitivity of TM2-PK increased when it was analyzed in combination as parallel tests with LDH and beta-2 microglobulin (86%), while the specificity was measured as 32%. In serial analysis, the specificity was increased to 98%, while the sensitivity was 5%. Statistically significant agreement in diagnosing MF was also noted between TM2-PK and LDH levels. TM2-PK may not be a useful marker for MF, especially in early-stage patients, because it proliferates slowly. We think that TM2-PK levels should be investigated in advanced-stage MF or in other types of cutaneous T-cell lymphomas; in particular, in combination with other established markers.