The X chromosome-linked scurfy (sf) mutant of the mouse is recognized by the scaliness of the skin from which the name is derived and results in death of affected males at about 3-4 weeks of age. Consideration of known man-mouse homologies of the X chromosome prompted hematological studies, which have shown that the blood is highly abnormal. The platelet and erythrocyte counts are both reduced and become progressively lower relative to normal as the disease progresses. There is gastrointestinal bleeding, and most animals appear to die of severe anemia. By contrast, the leukocyte count is consistently raised. Some animals showed signs of infection but it is not yet clear whether there is immunodeficiency. Other features include the scaly skin and apparently reduced lateral growth of the skin, conjunctivitis, and diarrhea in some animals. The mutant resembles Wiskott-Aldrich syndrome in man, which is characterized by thrombocytopenia, eczema, diarrhea, and immunodeficiency. The loci of the human and mouse genes lie in homologous segments of the X chromosome, although apparently in somewhat different positions relative to other gene loci. Scurfy differs from Wiskott-Aldrich syndrome in that scurfy males are consistently hypogonadal.