The concentration of lyso-PAF in serum from untreated patients with Japanese cedar pollinosis, a typical allergic disease, was significantly higher than that in healthy controls. It was also found that the lyso-PAF level in serum from Japanese cedar pollinosis significantly decreased after therapy with ketotifen, an anti-allergic drug. These results suggest that PAF might play an important role in Japanese cedar pollinosis and that the clinical effect of anti-allergic drug might be partially due to the anti-PAF action.