Within the last 18 months, we have noted the development of unexplained lymph node enlargement in otherwise asymptomatic patients with hemophilia. Because such changes are poorly understood and, in some patient groups, may be related to the acquired immunodeficiency syndrome (AIDS), we studied the enlarged lymph nodes in four patients with severe factor VIII deficiency and abnormally low peripheral blood helper-inducer/suppressor cell (OKT4/OKT8) ratios. Surgically excised lymph nodes were studied for histopathologic, electron microscopic, and chromosomal changes. Cell suspensions from these and normal nodes were also studied using monoclonal antibodies. Excised lymph nodes showed follicular hyperplasia. Electron microscopy revealed no viral particles or vesicular rosettes. Chromosomal aberrations included an acrocentric marker chromosome in one patient and monosomy 21 in another. T lymphocyte ratios (OKT4/OKT8) in lymph node suspensions were lower than those in nodes from normal controls (1.2 v 6.1) and reflected the lymphocyte ratio in peripheral blood. Mature B cell percentages were increased in the lymph nodes from patients with hemophilia (38% v 27% in controls). Patients treated with factor VIII concentrates and male homosexuals have similarities in persistent lymph node enlargement, histologic features of follicular hyperplasia, and changes in lymph node and circulating lymphocyte subpopulations.