The existence of chestnut pillions in Paris is not generally accepted in spite of the production of very large quantities of chestnut pollen in July. We carried out systematic skin-prick testing with chestnut pollen on 81 patients seen in the allergology department. The results were positive in 17 of the 47 patients presenting seasonal symptoms and in none of the 34 patients with perennial symptoms (x2 test= 12.12, P< 0.0). Eight of the 17 positive patients had sIgE levels above 300 IU/ml, 15 had a positive RAST and 14 had a positive nasal provocation test result. On the basis of these findings and the results of the clinical examination during the peak period of pollen emission, we diagnosed chestnut pollinosis in 11 patients. We draw our conclusions from the evidence of this form of pollinosis in the Paris region, together with the reasons why it is, as yet, rarely recognized.