Since rising serum lactate dehydrogenase (LDH) heralds progression in patients with lymphoma or myeloma we investigated the significance of its elevations during chemotherapy supported by granulocyte (G-) or Granulocyte-Macrophage (GM-) colony stimulating factors (CSF). To Exclude effects of resistant disease we analyzed 52 courses of therapy in 36 responding patients. During hematologic recovery LDH increased above normal in 53% and 85% of patients with leukocyte counts of 10,000/microL and 15,000/microL, respectively. After CSF discontinuation LDH fell to or towards normal during 20 courses with adequate follow-up. Therefore rising serum LDH in patients with lymphoma or myeloma may be caused the CSF administration during chemotherapy and not by progressive disease. Proper identification of this effect can prevent unnecessary tests or treatment delays.