Changes in the level of serum neutrophil chemotactic activity (S-NCA) were investigated in 20 subjects with allergic rhinitis, with or without asthma, undergoing clinically effective hyposensitization to house dust mite with Pharmalgen®Dermatophagoides pteronyssinus. Two control groups were studied: 28 subjects with allergic rhinitis, with or without asthma, receiving placebo injections for 1 yr in a double-blind controlled trial with Pharmalgen®D. pteronyssinus (from whom the actively, treated group in this study were recruited), and eight non-atopic asymptomatic controls. S-NCA and serum IgE specific to D. pteronyssinus were measured in the subjects before, during (3–6 months) and 12 months after treatment, and once in the non-atopic controls. The mean S-NCA was significantly higher (0.01 > P > 0.001) in subjects before treatment (mean±s.e. = 63.8 ± 3.6 arbitrary units of migration (AUM)) compared with the non-atopic controls (48.5 ± 3.7 AUM), but had fallen to normal levels after 6 months (46.8 ± 4.0 AUM) and 12 months treatment (45.2 ± 3.8 AUM). The levels of S-NCA in the placebo treated group were significantly higher than normal at the start of treatment (69.2 ± 4.1) and remained raised throughout the 12 months treatment. In the actively treated group, the level of S-NCA had fallen in 18 out of 19 subjects after 12 months immunotherapy, and was unaltered in one. Mean levels of D. pteronyssinus IgE rose during the first 6 months and declined to initial levels by the end of the treatment. We suggest that the elevated S-NCA may play a role in the maintenance of the allergic state through the induction of the late-phase reaction; that the abnormal S-NCA production is reduced by effective hyposensitization, and that measurement of S-NCA may be useful to monitor hyposensitization therapy. Complete characterization of S-NCA and the mechanisms of its control remain to be investigated.