Because an immunological defect of an unknown nature is thought to be a factor in the pathogenesis of pernicious anemia, we studied lymphocyte surface phenotypes in 40 patients and compared them with 113 healthy controls. The only significant difference that emerged was in their slightly decreased number and proportion of surface immunoglobulin (lambda chain) -bearing cells. Of specific interest, the numbers of OKT4+ and OKT8+ lymphocytes and the T4+/T8+ ratios were not significantly different from control values. A sizable minority of patients had increased ratios, while a smaller number had decreased values. No explanation or identifying feature was apparent for those patients with either increased or decreased T4+/T8+ ratios. (One patient with a very low ratio, who was excluded from our analysis, developed acquired immune deficiency syndrome.) No differences were apparent in T4+/T8+ ratios or any other lymphocyte surface phenotypic characteristics when patients were segregated by presence of anti-intrinsic factor antibody or anti-parietal cell antibody, or by race, sex, or age. Our results in a racially heterogeneous group of patients do not support the suggestion that T4+/T8+ ratios are usually abnormal in prenicious anemia or that the presence of anti-intrinsic factor antibody is associated with such abnormality.