Among 313 patients with serum IgG deficiency, selective IgG3 deficiency was found in 59.5%, combined IgG3 deficiency together with IgG1 deficiency in 36% and combined IgG3-IgG2 deficiency in only 4.5%. Most of the patients with IgG3 deficiency suffered from upper respiratory tract infections, but many also from recurrent bronchitis, bronchopneumonias and asthma bronchiale. Those with combined IgG3-IgG1 deficiency often suffered from obstructive lung disease and chronic lower respiratory tract infections. Other diagnoses found in patients with IgG3 deficiency were diabetes mellitus, Henoch-Schönlein, recurrent herpes simplex infections and recurrent erysipelas. IgG3 deficiency was also found in relatives of patients with common variable immunodeficiency and IgA deficiency. In one family both parents and all four children showed IgG3 deficiency, some of them also C2 deficiency. In another family 2 siblings with diabetes mellitus showed IgG3 deficiency. In still another family the mother and her daughter both with asthma bronchiale showed IgG3 deficiency. Patients with IgG3 deficiency could respond to pneumococcal vaccine and seemed to respond to immunoglobulin substitution given every or every other week.