We have recetitly phenotyped inflammation iti tion-infectious allergic and non-allergic chronic maxillary sinusitis using sinus biopsies and lavage fluids. In this first paper, we have concentrated our work on the eosinophil, T cell, mast cell and macrophage infiltrates. However, many unresolved questions remain and particularly the role of neutrophils needed to be addressed. In the present study, we focused on the neutrophilic inflammation: myeloperoxidase (MPO) and interleukin-8 (IL-8) were measured by immunoassays and neutrophils were enumerated by conventional staining in the sinus lavage fluids of 16 patients with chronic sinusitis and six control subjects. Both MPO and IL-8 levels were significantly higher in patients than in controls (P < 0.01 and 0.005, respectively). There was a significant correlation between MPO levels and neutrophil numbers, and between MPO and IL-8 levels in the sinus lavage fluid (P < 0.0001, Spearman rank correlation). The presence of high levels of IL-8 in the lavage fluids of patients suffering from chronic sinusitis, levels which correlate with those of MPO, suggests that this cytokine may activate neutrophils in this chronic disease.