We evaluated the safety of immunotherapy in 419 patients who attended our allergy department for treatment. They were suffering from rhinitis and asthma caused by sensitization to grass pollen or Dermatophagoides pteronyssinus. Immunotherapy was given by biologically standardized aluminum hydroxide adsorbed extracts according to a conventional schedule. Local reactions were recorded in 10.5% of the patients and systemic reactions in 4.8%. Only 0.37% of the doses administered were associated with systemic side-effects. We found that 84% of the patients who showed systemic reactions were asthmatic subjects (P<0.01), and most of them were sensitized to D. pteronyssinus (71%). Side-effects occurred more frequently during the dose-increase period (P<0.05). After 9482 doses had been administered, no anaphylactic shock or life-threatening reactions were registered. We believe the risk associated with immunotherapy to be drastically reduced when treatment is carefully monitored by skilled personnel. In such conditions, as shown by our study, immunotherapy is safe.