Studies of fetal hemoglobin and bone marrow ultrastructure were performed on a patient with a recently described syndrome consisting of lymphosarcoma, short stature, defective cellular immunity, hypogammaglobulinemia, and mosaicism for a marker chromosome. Abnormalities of the erythroid cells, including multinucleated erythroblasts, nuclear bodies, megaloblasts, and frayed cytoplasm, were seen by light microscopy. Ultrastructure confirmed the findings by light microscopy and showed in greater detail nuclear malformation, loss of nuclear membrane, and loss of cytoplasmic background, as well as peculiar cytoplasmic inclusions. An increased level of fetal hemoglobin (Hb F), ranging between 12%-15%, was detected. The Hb F was discreetly distributed, occurring in a small per cent of the red blood cells. It is speculated that a clone of chromosomally abnormal erythroid precursors might be responsible for producing the erythroblasts with the structural abnormalities described and the erythrocytes with the high hemoglobin F content.