Long term observations of atmospheric pollen offer the possibility of studying patterns in fluctuations in the airborne pollen presence. In this study, birch pollen measured at five monitoring stations in Europe with records of 18 to 30 years, have been analysed comparatively on three aspects: trends and fluctuations of annual sums of daily concentrations, and starting dates of airborne presence. The results show weakly rising trends at all five stations. The observed fluctuations in airborne birch pollen quantities are suggestive of synchronous, alternatingly cyclic rhythms, beit with a-synchronous aberrant years and phase shifts. For the starting dates of the birch pollen season the air temperature during the preceding 4 decades-of-days are of decisive importance.