Immunoglobulin M (IgM) provides the initial response to foreign antigen and plays a regulatory role in subsequent immune response development, accelerating the production of high-affinity IgG. Though selective IgM deficiency was described more than 45 years ago in children with fulminant meningococcal septicemia, it has been largely an ignored primary immunodeficiency. It appears to be more common than originally realized. Selective IgM deficiency is observed in both children and adults with no gender bias. The most common clinical manifestation of selective IgM deficiency is infections with extracellular and intracellular bacteria, viruses, and fungi. Allergic diatheses are the second commonest presentation of selective IgM deficiency. There is an increased prevalence of autoimmune diseases, which in both humans and mice appear to be secondary to selective IgM deficiency rather IgM deficiency secondary to autoimmune diseases. Selective IgM deficiency, in some cases, is associated with 22q11.2 chromosome deletion and few familial cases of selective IgM deficiency have been reported. Innate immunity is relatively intact. T cells, T cell subsets, and T cell functions are normal. However, several patients with selective IgM deficiency and T cell and NK cell defects with Mycobacterium avium intracellulare infections have been reported. In a subset of patients with selective IgM deficiency circulating IgM+ B cells are decreased or completely lacking. Specific IgG antibody responses against pneumococcus polysaccharides are impaired in a subset of patients with selective IgM deficiency. The pathogenesis of selective IgM deficiency is unclear; decreased T helper activity, increased isotype-specific suppressor T cell activity, and intrinsic B cell defects have been reported. Patients with selective IgM deficiency and impaired pneumococcal antibody responses appear to respond to immunoglobulin therapy.