Measurement of serum IgG subclass levels in 3005 patients disclosed abnormally low IgG1 levels with normal levels of the other IgG subclasses and of IgM and IgA in 119 patients, predominantly adults. Not all patients were hypogammaglobulinemic due to nonrare increases of other isotypes, mostly IgM. A familial context of immunodeficiency was frequent, more often combined than selective IgG1 deficiency. A familial association with IgG2 deficiency was found also and IgG1 replaced IgG2 deficiency in 3 cases (and succeeded to or preceded more complex IgG defects in 3 cases, whereas IgG1 deficiency was consistently found at examinations repeated in the absence of therapy in 10 additional cases). Most but not all (83.2%) patients suffered infections, generally moderate, similar to those observed in other selective IgG subclass deficiencies (IgGSD), with predominantly sinorespiratory infections. Other clinical manifestations such as atopy, congenital cardiopathy, and autoimmune diseases were already known in IgGSD but the incidence of asthma was strikingly high (one-fifth of the cases).