The purpose of this study was to evaluate the incidence, type and potential risk factors of systemic reactions due to conventional allergen immunotherapy with aqueous extracts. The study was carried out retrospectively and included 1,506 patients to whom allergy injections had been given during the last 12 years. Symptoms in each reaction were classified with respect to time of onset, involvement of respiratory tract or skin, and presence of hypotension. The results showed 125 systemic reactions in 109 patients (1 per 1,831 injections), of which 52.8% were of the skin, 12% were respiratory symptoms, 30.4% respiratory symptoms and skin, 0.8% hypertension only, and 4% hypotension with respiratory symptoms and skin reactions. Most of the systemic reactions (84.8%) occurred within the 30 min after injection. Forty-one per cent of the systemic reactions were observed in the build-up period (1/52 patients, 1/1, 158 injections) and 58.4% in the maintenance injection period (1/73 patients, 1/2,311 injections). Seventy-six systemic reactions were related to pollen season (60.8%), 11 were related to injection from a new vial (8.8%) and eight to both pollen season and a new vial (6.4%). Thirty-five per cent of the patients who experienced systemic reactions had bronchial asthma, but there was no correlation between frequency of the two. There was also no correlation between systemic reactions and age or sex. It was concluded that immunotherapy has a low rate of systemic reactions and that maintenance immunotherapy appears to be associated with fewer such reactions than the build-up period. The 30 min waiting period is adequate for conventional immunotherapy. However, a longer waiting period may be necessary for high-risk subjects.