Summary. In 100 untreated patients with chronic lymphocytic leukaemia (CLL) lymphocyte doubling time (LDT) has been investigated in relationship with clinical stages, bone marrow histological patterns, treatment-free period and survival. Although partially correlated with clinical stages and bone marrow patterns. LDT has a clear prognostic significance by itself: whereas a LDT of 12 or less months identifies a population of patients with poor prognosis, a LDT higher than 12 months is indicative of good prognosis as substantiated by a long treatment-free period and survival. This simple parameter can be useful in the clinical management of CLL patients.