Nine patients with a house dust mite (HDM) allergy were monitored for one and a half year starting in Spring 1983 during immunotherapy with aqueous alum precipitated HDM extract. Evaluation included nasal provocation tests with HDM extract and histamine chloride. Nasal responsiveness was assessed by measurement of the nasal airway resistance, by counting the number of sneezes and measuring the amount of secretion. During the one and a half year hyposensitization a decrease in nasal sensitivity to HDM extract is found when measurements are compared at yearly intervals (Spring 1983-1984 and Autumn 1983-1984). However, nasal reactivity to HDM extract is elevated in autumn compared with spring (not significant in 1983, but significant in 1984). Changes in nasal sensitivity to histamine are not so obvious except for the interval between Spring 1983 and Autumn 1983. The fluctuations in nasal sensitivity could not be attributed to baseline variation in nasal resistance during the trial. We conclude that seasonal variation in sensitivity to HDM can influence the results of immunotherapy with HDM extract, and should be considered when evaluating such treatment.