We describe 2 siblings who had interleukin- 1 receptor- associated kinase 4 deficiency with a novel mutation in exon 2. They had delayed separation of the umbilical cord. The flow cytometric analysis of monocytic intracellular tumor necrosis factor- α production in response to lipopolysaccharide may be a useful method to screen for the disease. We describe 2 siblings who had interleukin- 1 receptor- associated kinase 4 deficiency with a novel mutation in exon 2. They had delayed separation of the umbilical cord. The flow cytometric analysis of monocytic intracellular tumor necrosis factor- α production in response to lipopolysaccharide may be a useful method to screen for the disease.