While much is known about CML at both the clinical and molecular biological levels, the precise relationship between the disease at these two levels is unclear. The appearance of the fusion gene bcr-abl and disorders in the regulation of the myc gene, and perhaps other oncogenes which code for nucleoproteins, appear to play integral roles in the genesis of the chronic and blastic phases of the disease. The resistance of this disease to cytotoxic therapy appears to reflect both "classical" drug resistance and the ability of those cells which survive cytotoxic therapy to rapidly replace the killed cells thereby offsetting the effects of chemotherapy ("regrowth resistance"). The clinical evolution of the disease is compatible with two fundamentally different processes: one compatible with a deterministic chaotic model and the other involves two basically independent linear phenomena which overlap and intersect as the blastic phase appears and replaces the chronic phase.