The ultrastructure of the lymphocytes from three children with severe combined immunodeficiency disease (SCID) is described. Parallel tubular arrays (PTA) were found in a large percentage of circulating lymphocytes (53%, 41%, and 13%) in three SCID patients when compared to age-matched controls. The size of these inclusions was quite variable, with some attaining a length of 1.7 micrometer. They contained a tubular substructure with a diameter of 36--44 nm. The PTA were mostly located in the centriolar and Golgi regions of the cytoplasm, and were sometimes membrane bound. A centriolar origin of the inclusion was suggested. A second inclusion, the tubuloreticular structure, was found in only 1.4% of the circulating lymphocytes from one SCID patient. The origin of the PTA and its occurrence in severe combined immunodeficiency disease are discussed.