Plasma levels of cyclic nucleotides were measured in 71 patients with acute leukemia [50 patients with acute nonlymphocytic leukemia (ANLL), five patients with hypoplastic leuke-mia, and 16 patients with acute lymphocytic leukemia (ALL)], five patients with myelodysplas-tic syndrome (MDS), and 47 healthy volunteers. The cyclic GMP (c-GMP) level, cyclic AMP (c-AMP) level, and c-AMP/c-GMP ratio in healthy volunteers were 15.74±5.10 pmol/ml, 3.20±1.15 pmol/ml and 5.39±2.18, respectively. In the patients with untreated acute leukemia other than hypoplastic leukemia, c-GMP levels were significantly elevated (ANLL : 11.31±13.61 pmol/ml ; ALL : 10.66±7.23 pmol/ml) and c-AMP/c-GMP ratios were significantly reduced (ANLL : 2.01±1.03 ; ALl : 1.66±1.03). These values normalized at remission, than increased or decreased again with recurrence of the disease. Although patients with MDS showed normal c-GMP levels or c-AMP/c-GMP rations, these values were increased or decreased when progression to acute leukemia occurred. There was correlation between c-GMP levels and peripheral leukocyte counts (r=0.429,p<0.05), plasma c-GMP levels and peripheral leukemic cell counts (r=0.412,p<0.05), c-AMP/c-GMP rations and peripheral leukocyte counts (r=-0.577,p<0.05), c-AMP/c-GMP rations and periph-eral leukemic cell counts (r=-0.512,p<0.05), and c-AMP/c-GMP rations and periph-eral leukemic cell counts (r=-0.512,p<0.05), and c-AMP/c-GMP rations and maximum counts of colonies derived from leukemic blast progenitors (r=-0.996,p<0.01). Since plasma levls of cyclic nucleotides reflect the leukemic cell proliferation, measurement of these nucleotides was considered useful for monitoring leukemic cell volume.