The majority of patients with congenital clotting disorders who use clotting factor concentrate exhibit lymphocyte subpopulation abnormalities. A subset of these patients develop lymph node enlargement (LNE), part of the spectrum of clinical disease associated with the acquired immune deficiency syndrome (AIDS). It is therefore important to determine if these patients with LNE exhibit specific immune alterations suggestive of early infection with the AIDS agent. We used one- and two-color immunofluorescence to distinguish the lymphocyte subpopulation alterations associated with concentrate use from those associated with LNE. Patients who use concentrate had elevated levels of Leu-2+ (T suppressor phenotype) cells and Leu-7+ (phenotype of some natural killer) cells. These increased levels were largely caused by a dramatic (2.6-fold) increase in the number of lymphocytes co-expressing Leu-2 and Leu-7 (2+7+). A dose-response effect between amount of concentrate infused during the preceding year and level of 2+7+ cells was observed. Concentrate recipients, as a group, also showed increased levels of T cells expressing Dr antigen (T+Dr+ phenotype, characteristic of activated or immature T cells) and cells expressing the T10 antigen (phenotype of some null cells and activated/immature T cells). Patients with LNE showed a further increase in T10+ cells as well as a distinctive decrease in Leu-3+ (T helper phenotype) lymphocytes. All LNE patients exhibited either low Leu-3+ levels, high T10+ levels, or both. Thus, concentrate use was associated with increased levels of Leu-2+ (particularly 2+7+) cells and T+Dr+ cells, whereas LNE was associated with decreased levels of Leu-3+ cells and high levels of T10+ cells.