The author reviewed 53 cases of birch pollinosis treated in our ENT-clinic between 1990 and 1991. Of interest is the fact that many of these patients complained of typical symptoms in and after June, when birch pollen has usually disappeared. We considered this discrepancy to be attributable to the effects of grass pollen, one of the common causal agents in this period. However, only a few patients tested positive for grass pollinosis. We therefore postulated that cross-reactivity between birch and oak pollen accounted for this phenomenon, because both trees belong to same order, Fagales. A strong correlation between RAST scores for birch and oak pollen was detected, and the oak pollen RAST score was significantly reduced by birch pollen extract in an inhibition test. These findings demonstrated actual cross-reactivity between birch and oak pollen. Birch pollinosis patients in Sapporo therefore have allergic symptoms from March to July.