Serum total immunoglobulin (G, A, M and E) and IgG subclass levels were studied in 23 patients with brittle asthma, 23 age- and sex-matched patients with mild asthma and 33 patients with recurrent infective exacerbations of their asthma. Patients with brittle asthma showed significantly reduced levels of IgG (mean ± SD, 8·8 ± 3·3 g l⁻¹) compared to patients with mild asthma (11·0 ± 2·5 g l⁻¹) (P<0·008) with further significant reductions in the brittle compared to the mild group in IgG₁ (5·2 vs 6·3, P=0·035), IgG₂ (2·4 vs 3·25, P<0·006), IgG₃ (0·39 vs 0·55, P<0·05) and IgA (1·91 vs 2·38, P<0·03). There were no significant differences between the brittle group and the group with recurrent infective exacerbations for any parameter, but the latter group showed significantly reduced levels of IgG (8·2, P<0·001), IgG₁ (4·9, P<0·00001) and IgG₂ (2·5, P<0·02) compared to the mild group. In all groups, there was no relationship between dose of inhaled steroids and levels of any antibody. These findings suggest that the presence of a mild degree of humoral immunodeficiency relates to severity of asthma, and suggests that immunoglobulin replacement therapy may be appropriate in patients with the more severe forms of asthma.