Chronic lymphocytic leukemia (CLL) has a variable clinical course. CD38 expression and IgV_H gene mutational status are independent predictors of prognosis, but their relationships and the CD38 cutoff level are unknown. Using cytofluorography, we analyzed CD38 in 148 patients, in 108 of whom we were able to evaluate IgV_H mutations, make correlations with disease history, and assess cumulative survival. Three different patient groups were identified by the CD38 expression pattern: a group homogeneously CD38⁻, a group homogeneously CD38⁺, and a group characterized by a bimodal profile, because of the concomitant presence of variable proportions of 2 distinct populations, one CD38⁺ and one CD38⁻. In CD38 bimodal expression patients the CD38⁺ subset was significantly more represented in the bone marrow than in the peripheral blood. For IgV_H mutations, 11.4% of CD38⁻, 84.6% of CD38⁺, and 68.0% of CD38 bimodal expression patients had no mutation. CD38 expression, IgV_H mutational status, and traditional prognostic factors were concordant. The progression rate was 12.9% for CD38⁻, 75.0% for CD38⁺, and 63.3% for CD38 bimodal expression patients. Only 25.8% of the CD38⁻patients but 63.3% of the bimodal and 75.0% of CD38⁺patients were treated. The presence of a CD38⁺ population, albeit small, correlated with the development of autoimmune manifestations. The CD38⁻ group has not yet reached the median survival, which is 183 months in the CD38⁺ group and 156 months in the CD38 bimodal expression group, regardless of the size of the CD38⁺ population. The presence of a distinct CD38⁺ population within the leukemic clone, rather than a numerical cutoff definition, correlates with IgV_H gene mutational status and, irrespective of its size, identifies CLL patients who will have progressive disease.