Clinical staging systems represent an important advance in predicting the course speed of CLL. Clinical staging systems do not, however, offer information with respect to the speed of evolution of the disease. Lymphocyte doubling time (LDT) is a simple parameter that is useful in arriving at a valid prognosis in CLL. Whereas a high LDT (greater than 12 months) identifies a population with a very good prognosis (median survival, not reached), a low LDT (less than or equal to 12 months) is associated with a poorer survival (median survival, 58 months). In addition, a short LDT predicts rapid disease progression in patients in the early clinical stages.