Hairy cell leukemia (HCL) is a usually chronic B cell lymphoproliferative disorder. To evaluate the prognostic significance of the various heavy and light chain determinants of the surface immunoglobulins (slg), we analyzed the clinical data and immunologic phenotype of 64 patients with HCL. Sixty-two of the 64 patients showed slg, which was invariably of only one light chain type (kappa 33, lambda 29). The actuarial survival of the cases expressing kappa-light chains was significantly better than those with lambda-light chains (p less than 0.002). This difference persisted when only cases with gamma or alpha gamma heavy chains were considered. No differences between the kappa and lambda-subgroups were discovered with respect to parameters of clinical importance. The various heavy chain classes of slg did not correlate significantly with the survival time. These results suggest that the immunologic phenotype, in particular the light chain type, may be a prognostic factor in patients with HCL.