We have analyzed 117 younger patients with chronic lymphocytic leukemia (CLL) (mean age, 44.5 years; SD, 4.8; range, 19 to 49; male/female ratio, 2.08) with three main objectives: (1) to see whether these patients have distinctive presenting clinical features; (2) to investigate the impact of the disease on survival; and (3) to analyze whether already well-known prognostic factors are also useful when applied to these patients. As compared with an older age population (greater than or equal to 50 years), there were no major differences in presenting features except for an increased proportion of males (2.08 v 1.21; P less than .025) and a higher hemoglobin level (13.47 +/- 2.70 g/dL v 12.84 +/- 2.77 g/dL; P less than .05) in the younger group. Median survival is 12.3 years (expected median from a control group, 31.2 years). Clinical stages, bone marrow patterns, blood lymphocyte counts, and its doubling time are all useful to separate different risk groups of patients. Whereas patients with favorable prognostic factors have a survival probability of about 80% 14 years after diagnosis, those with poor prognostic features have a median survival of less than 3 years. It is concluded that CLL in younger adults has no major distinctive presenting features and that known prognostic factors are useful to separate different risk groups of patients. These results should be of help in planning therapy for younger persons with CLL.