Thirty-nine children with birch pollinosis were given immunotherapy (IT) for 3 years with a purified, characterized and biologically standardized pollen preparation made from either birch or a mixture of birch, alder and hazel. Levels of IgE antibodies against birch, alder and oak increased early during IT (P < 0.01) and then decreased below the initial level (P < 0.01). Birch-specific IgG antibodies also increased (P < 0.01) but not as rapidly as birch-specific IgE antibodies and the levels remained increased throughout IT. There were only weak correlations between immunochemical findings before IT or during early IT on one hand and the improvement during IT on the other hand. The ratio birch-specific IgG: IgE after 1.5 and 2.5 years of IT correlated with symptom scores the following season (P < 0.01). In 60% of the children below 13 years of age, new IgE specificities developed during IT as demonstrated with crossed radio-immunoelectrophoresis. No such new IgE specificities were found in older children. These ‘new sensitivities’ did not appear to have any clinical implication.