SummaryA prospective study was made in France to determine the frequency, nature, causes and consequences of systemic reactions occurring during specific immunotherapy. One hundred and fifty five reactions were recorded in 151, 997, injections given to 19,739 patients; a percentage of 0.1. It was higher with pollen extracts (0.2%) and practically nil with other extracts (house dust, Dermatophagoides, insect body, bacteria). Asthma, spasmodic rhinitis and urticaria were the most frequent, 80% of systemic reactions. In 59% no explanation could be found. The main known causes of adverse reactions were excessive doses of antigen, improper timing of treatment or incorrect technique of injection. After appropriate treatment the immunotherapy was continued in nearly 90% of the cases. Specific immunotherapy with the majority of extracts now being used, namely adsorbed extracts, is not dangerous but it must be precisely and cautiously done because errors are responsible for nearly 50% of the recorded systemic reactions.