Alnus pollen is an early component of the annual atmospheric aerosol of the north-west regions of Spain, which causes the first occurrence of allergic symptoms. Seasonal and intra-daily variation of Alnus pollination, and the influence that main meteorological parameters exert, was studied in this paper. Monitoring was carried out from 1993-2002, by using two Lanzoni VPPS 2000 volumetric samplers. Once the atmospheric behaviour of this pollen had been identified, the final objective was to elaborate predictive models to determine the onset of the Alnus pollen season and its concentrations during the pollination period in two localities of north-west Spain (Santiago and Ourense). Winter chilling required to overcome the bud-dormancy period was similar in both cities, with around 800 Chilling Hours (C.H.) and 5.5 degrees C threshold temperature. Calculation of heat requirement for bud growth was carried out with maximum temperature, with around 50 Growth Degree Days (G.D.D. degrees C) needed, with 6 degrees C threshold temperature. Data from 2002 were used in order to determine the real validity of the models. This year was not taken into account to establish the aforementioned models. The variation between the predicted start of the pollen season and the observed season was smallest in Ourense. Verifying the proposed models for predicting daily mean concentrations of Alnus pollen during the pollen season shows that the predicted curves fits the observed variations of daily mean concentrations.