House dust mite (HDM) allergy is a major cause of allergic rhinitis and asthma in Sydney, however the link between the upper and lower airway symptoms experienced by rhinitics is not fully understood. Aim: To determine if there is seasonal variation in the severity of nasal symptoms and to determine if the occurrence of bronchial symptoms is associated with high nasal symptoms. Methods: This was a parallel prospective cohort study of 35 perennial allergic HDM sensitised rhinitics and 19 healthy non-allergic controls. Nasal and bronchial symptoms were recorded every fortnight by telephone questionnaire. High nasal symptoms were defined as those above the normal range, which was calculated from the control group (mean + 2SD). For each subject, in each season, we calculated the percentage of fortnightly records at which a) nasal symptoms were high, b) bronchial symptoms were present and c) the percentage of bronchial symptoms that occurred when nasal symptoms were high. One way ANOVA for repeated measures was used to test for seasonal differences. Results: Table: Symptom occurrence over one year by season (95% CI) Summer Autumn Winter Spring P-value a) 63% (±11) 54% (±11) 53% (±10) 73% (±11) 0.006 b) 28% (±10) 25% (±9) 29% (±9) 32% (±10) 0.22 c) 47% (±16) 32% (±14) 37% (±14) 48% (±16) 0.13 Conclusion: Rhinitics experienced significant seasonal variation in nasal symptoms. However, there was no seasonal variation in bronchial symptoms, and no significant seasonal increase in the proportion of bronchial symptoms occurring in the presence of high nasal symptoms.