One hundred sixty-four patients with allergic rhinitis (104 patients with positive late nasal responses (LNRs) and 60 patients with negative nasal responses to allergen challenge), correlating with history and skin tests, were randomly selected for this study. Nasal challenges were repeated and supplemented by cytologic examination of the nasal secretions (NSs). The cytologic examination of NS was also performed during 164 control challenges with phosphate-buffered saline. The positive LNR was accompanied by significant changes in the count of neutrophils in 84% of the cases (increase immediately before and decrease during appearance of LNR, with increase again during resolving of late nasal response), of eosinophils in 58% (increase immediately before and decrease during appearance of LNR), of epithelial cells in 73% (increase followed by decrease, running parallel with LNR), of goblet cells in 63% (increase followed by decrease), of basophils in 8%, and of lymphocytes in 6% (both of these cells demonstrated a slight increase during LNR) in the NS. No significant changes in the count of other types of cells in the NS during most of the LNR cases were recorded. Most cases of negative response were not accompanied by significant changes in the count of individual cell types. No significant changes in the count of any cell type in the NS were recorded in any patient during the phosphate-buffered saline-control challenges. The cytologic examination of NS, accompanying the nasal challenge with allergen, appears therefore to be a valuable supplementary diagnostic parameter for the LNR and a promising model for further immunologic and clinical studies of the LNR.