There are some data suggesting that the natural history of chronic lymphocytic leukemia (CLL) may be changing, but a systematic analysis of this topic is lacking. To address this issue, we examined two cohorts of CLL patients in whom the diagnosis was established in 1960-1979 (group I) and in 1980-1989 (group II), respectively. Striking differences were observed between both cohorts. The diagnosis in the second group was established at higher age (65.8 vs 61.3 years; P = 0.0001), both in males (63.8 vs 59.1 years; P = 0.004) and females (68.3 vs 64.2 years; P = 0.01); the proportion of patients in whom the diagnosis was established in low-risk clinical stage (Binet's A) was significantly higher in group II (65.7% vs 42.6%; P < 0.001), and the survival was more than double in group II (median of 11.1 vs 5.3 years; P < 0.0001). Moreover, the impact of the disease on life expectancy was much lower in the more recent cohort. These differences may be due, at least in part, to changes in the natural history of the disease.