It has been suggested that the season of birth is a predisposing factor for allergic sensitization. In the last few years many authors in different countries have found a statistically significant correlation between the month of birth and the subsequent allergy for grasses and birch pollens and house dust mites. Nevertheless, other investigators failed to confirm these results. The aim of the present study is to assess the relationship between the season of birth and the development of respiratory allergy to pollen of Parietaria, an Urticacea plant characteristic of Mediterranean flora, which is the most important hay fever producing one in our country. Among the patients who visited the Servizio Autonomo di Allergologia at the St. Martino Hospital (Genoa, Italy) in the year 1986, 264 subjects (113 F and 151 M, 5-35 years of age, all born and living in Genoa and its suburbs) were selected. The birth month of these patients, affected by rhinitis and/or bronchial asthma and with positive prick-test only toward Parietaria pollen, was recorded and compared with that of the population born in Genoa in the same years. Statistical evaluation made by X2 test showed the absence of a statistically significant relationship between the birth in the pollen season (April-July) and the pollinosis, with the exception of the month of May. The reasons of the discrepances observed between the behaviour of Parietaria pollen and that of the other pollens, grasses and birch, described in the above studies, are discussed.