Untoward allergic reactions were monitored prospectively in 98 patients undergoing hyposensitization with depot type preparations of house dust mites (42 patients) or grass pollens administered before the pollen season (56 patients). 44% of the patients experienced at least one major local reaction, usually when injected with high doses of allergens. A mild systemic reaction (benign crisis of rhinitis or asthma within 12 hours after the injection) was reported at least once by 9% of the pollen-treated and 52% of the mite-treated patients. A severe systemic reaction, which improved rapidly with adrenaline, was noted in 3% of the pollen-treated and 5% of the mite-treated patients. These data indicate that untoward allergic reactions during hyposensitization are not uncommon, and this type of treatment therefore requires a strict indication.