We report our experience of adverse reactions to immunotherapy (IT) in patients with insect venom allergy and inhalant respiratory allergy. Adverse reactions included large local reactions, generalized cutaneous reactions or systemic reactions. Among 87 patients treated for venom allergy, 43% had adverse reactions during the course of IT, averaging 2.5 reactions per patient and per course of IT. Nine had systemic reactions, of which 7 required adrenaline administration. Among 52 patients treated with inhalant allergen extracts, 40% had adverse reactions averaging 3 reactions per patient per course of treatment. Ten patients had systemic reactions but only 2 required adrenaline administration. There was no difference between the rate of adverse reactions in the venom and the inhalant treatment groups. IT has an inherent risk which has to be weighed against its benefits.