Objective. To describe a novel syndrome characterized by severe prenatal and postnatal growth failure, mild skeletal and facial abnormalities, and primary immunodeficiency. Design. The syndrome was observed in 2 sisters. The elder child died of cytomegalovirus infection when she was 18 months old, whereas the younger sister is doing well at 5 years old. We report here clinical, hematologic, and immunologic data for both sisters and compare them with all known inherited disorders with similar clinical or immunologic features. Results. The immune defect consists of a lack of detectable natural killer cells and small numbers of CD8 αβ T cells and polymorphonuclear neutrophils. This is the first report of prenatal and postnatal growth failure associated with mild skeletal and facial abnormalities and primary immunodeficiency. Conclusion. This novel syndrome probably is caused by an autosomal recessive gene defect impairing both intrauterine growth and natural killer cell development. The identification of other kindreds with this syndrome would facilitate the search for its genetic basis.