— A mathematical model that represents the coexistence of normal and leukemic populations of cells is introduced and analyzed. It is a nonlinear time-delay system characterizing the dynamics of ordinary stem cells, coupled to a delay differential-difference system governing the dynamics of mutated cells. An important feature of this model is that it may explain the invasion of the bone marrow by unhealthy cells that are characterized by fast self-renewing dynamics. A Lyapunov-like technique is developed to investigate the stability properties of a steady state where the population of healthy cells will survive. Exponential stability of solutions is established, estimate on their decay rate is given and an open subset of the basin of attraction of the studied equilibrium is provided.