Nasal challenges with pollen grains are as close as possible to natural pollen exposure, but they are not well documented in grass pollen allergy. Forty-four grass pollen allergic patients and ten non-allergic volunteers were tested by means of nasal challenge, quantitative skin-prick tests with a standardized orchard grass pollen extract and serum-specific IgE. Nasal challenges were performed with lactose and increasing concentrations of orchard grass pollen grains (15-3645 grains, three-fold increase). The test was considered to be positive when a symptom score over 5 was obtained, since this score had been previously correlated with the release of PGD2 in nasal secretions. All control subjects and 3/44 patients had a negative challenge. The number of orchard pollen grains required to elicit a positive challenge was 332 +/- 440 (range: 15-1215 grains) and the distribution was Gaussian. This number is higher than expected according to pollen calendars performed during the season, but owing to the priming effect of the nasal mucosa by allergens it is compatible to natural exposure. The correlation between nasal provocation tests and skin-prick test end-points was significant (P less than 0.005, Spearman rank test). Conversely there was no correlation between nasal challenge or skin-prick test end-point and serum-specific IgE.