Thirty children with rhinoconjunctivitis due to birch pollinosis were treated in a double blind manner for 10 months with enteric-coated capsules containing either a birch pollen preparation (n= 14) with doses up to 1.4 × 106 biologic units per day or placebo (n= 16). Compared with the placebo group the actively treated children had less symptoms during the birch pollen season after 3 months of therapy (P = 0.035). Skin prick reactions decreased significantly more in the active group than in the placebo group after 10 months (P = 0.01). Conjunctival sensitivity was lower in the active group than in the placebo group after 3 months of treatment (P = 0.01) but not after 10 months. Compared with the placebo group treated children more often increased their levels of IgG (P= 0.007) and pre-seasonal IgE (P= 0.001) against birch. There was a seasonal increase of IgE antibody level against birch in the placebo but not in the treatment group (P < 0.001). None of the treated children developed asthma, compared with five of the untreated children. No general reactions occurred and few side effects were seen during the treatment period. We conclude that in children with birch pollinosis oral immunotherapy with high doses of a biologically potent preparation in enteric-coated capsules is effective, easy to perform, economic and safe.