Background: The history of seasonal, IgE-mediated (allergic) rhinitis is often obtained postseasonally as a retrospective assessment. It is not known whether recollected drug use is representative for the in-seasonal medication history. Objectives: To investigate the agreement between in-seasonal and out-seasonal reports on drugs and drug doses, and to investigate whether retrospective assessment was sufficiently sensitive to detect the effect of intervention. Material and methods: Thirty-five patients allergic to birch pollen were allocated to allergen-specific immunotherapy (SIT) or placebo in a double-blind study. Drug use was recorded daily during the season 2000 and out-seasonally 6 months after the seasons 1999 and 2000. The mean daily drug use was transformed into a 4-point categorical scale for simplicity and was calculated for the out-seasonal rating and for 6 in-seasonal periods of different durations. In-seasonal and postseasonal ratings were compared with the Cohen weighted κ (κw). Results: We observed a high level of agreement between in-seasonal and retrospective reports of drugs used (κw > 0.60) and drug doses (κw > 0.50). The highest agreement was observed for the in-seasonal day, week, or 2 weeks with the highest drug intake. Out-seasonal ratings significantly overestimated the daily oral and nasal antihistamine use compared with in-seasonal periods > 2 weeks. Despite being comparable pretreatment, only the SIT group had a significant decrease in recollected total drug use during SIT. Conclusion: Out-seasonal recollection of hay fever drugs represented a period of a maximum 14 days. It appeared sufficiently sensitive to detect the effect of intervention.