Flow cytometry (FCM) was used to monitor the cell cycle changes which occurred in the bone marrow cells of a patient with acute myeloblastic leukemia following pre-induction treatment with hydroxyurea. Flow cytometric and cytogenetic analysis showed two populations of cells; one hypertetraploid and presumably leukemic, and the other diploid and possibly normal. Sequential FCM monitoring of bone marrow cells after hydroxyurea demonstrated an early rise (after 24 h) in the S-phase component of diploid cells and a rise in the S-phase component of hypertetraploid cells 48-72 h later. Conventional induction therapy timed to coincide with this latter peak resulted in early remission.