Peripheral blood lymphocyte subsets were enumerated in five children with abnormal lymphatic circulation (three with lymphangiectasia, one with chylothorax, and one child with chyloperitoneum). All patients were lymphopenic. The percentage and absolute number of blood T-lymphocytes (CD3) were low in two children and normal in the other children. The percentage and absolute number of helper/inducer lymphocytes (CD4) were markedly reduced in all patients. The percentage of suppressor/cytotoxic lymphocytes (CD8) was normal or elevated in all children, and the absolute number of CD8 cells was normal in three patients. The CD4/CD8 ratio was reversed in all patients. In the two patients tested, the proliferative response of peripheral blood mononuclear cells to mitogens was reduced. T-lymphocyte subsets were measured in the pleural or peritoneal fluid of three patients, and the CD4/CD8 ratio was normal or increased. In each child, the CD4/CD8 ratio in the lymphatic fluids was markedly higher than the CD4/CD8 ratio of the blood (4.0/0.45, 1.75/0.95, and 1.3/0.85). The reversed CD4/CD8 ratio in the blood in cases of chronic loss of chyle may be due to either selective transport of CD4 lymphocytes into the lymphatic fluids or a shorter half-life of CD8 compared to CD4 lymphocytes. This finding may in part explain the abnormal cellular immunity previously observed in patients with lymphangiectasia.