Forty patients with tree pollen-induced allergy (rhinitis, conjunctivitis or combination from both with asthma) were hyposensitized with an extract from isolated birch pollen or a pollen mixture (hazel-, alder-, oak- and hornbeam pollen). The distribution of the different extracts was random. After 3-year treatment, no statistical differences were found between the two groups, especially regarding clinical effectiveness, duration of symptoms or symptom score. In conclusion, we recommend a hyposensitization with birch pollen extract in tree pollen-induced allergy. If evidence is present of sensitization against other tree pollens, especially hazel or alder, or in cases without therapeutic effect after two years of hyposensitization with birch pollen, an attempt with a tree pollen mixture extract is indicated.