Several in vitro studies have shown that the cyclin A gene is expressed and plays an important role in both the S and G2-M phases of the cell cycle. We analyzed cells from the blood and bone marrow of patients with various, mostly neoplastic, hematological disorders to determine whether (a) the cyclin A protein level correlated with that of cyclin A RNA and (b) cell distribution among the different phases of the cell cycle correlated with cyclin A RNA expression. Thirty-eight patients were studied by means of dot blot and Western blot techniques for cyclin A RNA and protein accumulation, and 21 were also studied for cell cycle distribution by using flow cytometric analysis. Semiquantitative studies were based on densitometric computerized evaluation of dot and Western blots. There was a very strong positive correlation between cyclin A RNA and protein expression (r = 0.99; P < 0.00005), indicating that cyclin A accumulation is regulated in these cells at a transcriptional level. There was also a highly significant positive correlation between cyclin A RNA expression and the cumulative percentage of cells in S plus G2-M phase (r = 0.98; P < 0.00005). Therefore, this in vivo study shows that the expression of cyclin A RNA and protein in human hematological malignancies correlates with the percentage of cells in S plus G2-M phase and identifies cyclin A as a new potential cell proliferation index in oncology.