Sixty-three adults with symptomatic chronic rhinosinusitis had computerized tomographic (CT) scans of the paranasal sinuses, which were used to quantify disease severity. These patients were divided into three closely age- and sex-matched groups: a CT scan-negative group (chronic rhinitis only), a mild sinusitis group and a severe sinusitis group. Serum dust mite-specific IgG levels were found to be significantly elevated in the sinusitis patients compared with a matched group of asymptomatic normal individuals. Levels were highest in the more severe sinusitis group, in which the mean titre was 559 U/ml and the incidence of titres greater than 400 U/ml was 48%, as compared with a mean titre of 139 U/ml and only a 10% incidence of titres greater than 400 U/ml in the normal subjects (P < 0.005 and < 0.01). In contrast, although the frequency of immediate hypersensitivity to dust mite, as assessed by intradermal skin tests, tended to be higher in patients with sinusitis, it was not significantly different from normal individuals. The association between mite IgG and disease was most striking in the patient sub-group with negative mite skin tests. In this group, mite IgG levels were significantly higher than normal, even in those patients with only chronic rhinitis. These findings demonstrate that increased serum levels of IgG against dust mites are strongly associated with chronic rhinosinusitis, especially in the sub-group of patients who are not allergic to mites.