Cartilage-hair hypoplasia is a short limbed skeletal dysplasia associated with impairments in host-defense. To better understand the clinical heterogeneity of this disorder, we studied 25 Amish patients with homozygous mutations in RMRP (RMRP 70 A>G). Despite mutation homogeneity, eight (32%) patients had severe or recurrent infections, two (8%) of these children underwent bone-marrow transplantation for combined immunodeficiency, and the remainder were healthy. Features distinguishing patients who underwent bone marrow transplantation from others were shorter birth length, and lower serum IgG, undetectable serum IgA, and elevated circulating NK cells before 2 years of age. Irrespective of clinical phenotype, most patients had lymphopenia and reduced lymphocyte proliferation to mitogens in vitro. Our cohort analysis suggests that many patients with cartilage-hair hypoplasia are at risk for infection susceptibility particularly during the first 2 years of life. Gauging this risk is difficult, and thus careful monitoring of all patients with cartilage-hair hypoplasia is warranted.