Of 94 hairy cell leukemia (HCL) patients studied for immunologic phenotype of their hairy cells, 89 patients had B cell markers and five patients were both surface immunoglobulin (slg) negative and E rosette negative. Forty of the 77 cases that had the heavy chains individually determined had IgG only (52.0%); 23 others had IgG in addition to other Ig heavy chains. Seventy-nine patients had monoclonal light chains; 65 with kappa chain and 14 with lambda chain. The only significant difference with respect to survival among the various slg groups occurred between the kappa chain and the lambda chain groups. Within the first 46 months after diagnosis of HCL, 20 deaths occurred among the 65 kappa chain patients, whereas the first and only death among the lambda chain patients occurred at 68 months after diagnosis. The only clinical or laboratory parameter that was significantly different between these two immunologic subgroups was the incidence of infections. Among the lambda chain patients, an infectious complication rate of 28.6% was observed subsequent to the diagnosis of HCL, whereas this rate was 68.8% in kappa chain patients (P = .005). The survival of lambda patients was found to exceed that of the kappa patients by the generalized Wilcoxon test (P = .03). However, when the log rank test was used, no significant difference was detected (P = .13).