Levels of 2′ and 3′ purine and pyrimidine ribonucleoside monophosphates (2′-, 3′-NMP) in leukocytes from blood and/or bone marrow were measured in three adult patients with acute non-lymphoblastic leukemias. The measurements of 2′-, 3′-NMP were made by high-performance liquid chromatography (HPLC) at various times in the course of the disease. Complete remission (CR) was obtained for all three patients but two of these have since died after relapsing at 8 and 9 months, respectively. The third patient remains in CR at 1½ year. The levels of 2′-, 3′-NMP in the leukocytes of the patient remaining in remission have not changed since the beginning of his remission. However, in the patients who relapsed 2′- and 3′-NMP levels increased first in bone marrow then in blood leukocytes. These increases occured about 3 months before the relapse was detected by morphological criteria. These data suggest that 2′-, 3′-NMP measurements may have a prognostic value if used to monitor patients with acute myeloid leukemia in CR.