We studied the incidence of IgG subclass deficiency in children with recurrent bronchitis. Recurrent bronchitis was defined as three or more episodes a year during at least 2 consecutive years, of bronchopulmonary infection, productive cough with or without fever and/or diffuse râles by physical examination in the absence of asthma or atopy. Fifty three children were selected, of whom 30 (57%) were deficient in one of the IgG subclasses. None had an IgG1 deficiency. Nine (17%) were deficient in IgG2, 9 (17%) in IgG3 and 20 (38%) in IgG4. Isolated IgG subclass deficiencies were most frequently seen for IgG4 (14, 26%), less for IgG3 (6, 12%) and even less for IgG2 (4, 7%). Nine (17%) children were IgA deficient and 8 (15%) IgG deficient with a combined IgG subclass deficiency in 8 and 7 of them respectively. By subdivision into different age groups most patients were encountered in the youngest group. The mean content of IgG2, IgG3 and IgG4 in 3- to 4-year-old children with recurrent bronchitis was significantly lower than in the age matched controls. The mean value for IgG4 in the 5- to 6-year-olds was significantly lower than in the control group. This study demonstrates the correlation between recurrent bronchitis in childhood and IgG subclass deficiency. IgG subclass deficiency and recurrent bronchitis are both quite prominent phenomena in young children but rare in older children, suggesting a transient immaturity of the immune system as one of the possible pathogenetic factors. An IgA or an IgG deficiency is highly suggestive for the existence of a combined IgG subclass deficiency.