Seventy-six consecutive untreated patients with B cell chronic lymphocytic leukemia (B-CLL) and classified according to Binet's staging system were studied at the clinical presentation. Several immunologic parameters (number of total and T circulating lymphocytes and their surface membrane immunoglobulin [Smlg] phenotypes and levels of serum Ig) were evaluated with the aim of identifying a biologic marker of prognostic relevance. In this series of persons, Binet staging confirmed its usefulness as a prognostic index (P less than .001). With regard to Smlg, they were mu-type in 41 cases (53.9%), mu-type plus delta-type in 29 cases (38.2%), alpha-type in one case, and not detectable in five cases. No correlations were found between clinical stage and immunoglobulin phenotype, although all but one patient in stage C showed mu-type Smlg alone. On analyzing the survival curves of our patients according to different Smlg phenotypes, we found that patients with only mu-type Smlg had a poorer prognosis (P less than .05) than those with mu-type plus delta-type; this difference was even more significant (P less than .01) in patients in stage A, whereas there were no statistical differences in those in stages B and C. Because the appearance of surface heavy chain of delta-type could be an expression of cell maturation, these results suggest that in B-CLL the presence of phenotypically more mature leukemic cells may correlate with better clinical prognosis, particularly in the early phase of the disease.