Thirty-nine children with rhinoconjunctivitis due to birch pollinosis were given immunotherapy for 3 years with a potent, purified pollen preparation made from either birch alone or from a mixture of birch, alder and hazel. The therapy was evaluated with self-evaluation after each season, daily symptom score during the season, conjunctival provocation test and skin-prick test. All children but one considered that their condition had been improved by the treatment. Symptom scores decreased slightly more in the group treated with the mixture than in the group treated with birch only due to symptoms decreasing faster in the mixture treated group after each pollen peak (P< 0.001). The sensitivity of the conjunctiva and the skin decreased significantly in both groups but without any significant differences between the groups. After 3 years the conjunctival sensitivity correlated strongly to eye symptoms during the season. Immunotherapy with a mixture of birch, alder and hazel appears to be at least as effective as a preparation made from birch only when treating birch pollinosis even in a region where birch is totally dominating among the pollens from deciduous trees.