We have previously reported a correlation between the extent of ragweed allergen (RWA)‐induced in vitro serum complement activation and the symptom scores registered daily during the ragweed (RW)‐blooming season in RW‐allergic patients. The present study was performed in 22 15–17‐year‐old RW‐allergic adolescents. Serum samples were incubated with 100μ/ml RWA, and the generation of different complement activation products was measured by ELISA or RIA. Symptom scores were registered for 4 weeks during the RW‐blooming season. The patients were divided according to the extent (low or high) of the generation of complement activation products, and symptom scores registered in the two groups were compared by two‐way ANOVA. Significantly higher symptom scores were obtained in the high than in the low complement activation group (P values: 0.049 for C1rC1sC1inh, 0.022 for CSbBbP, 0.015 for C5b‐9, 0.0001 for C3a, and 0.0008 for C5a). Similar results were obtained at the measurement performed in the sera obtained from the same patients half a year before the season (P values: 0.022 for C3bBbP, and 0.005 for C5b‐9). These findings indicate that complement activation induced by the allergen may enhance the clinical symptoms of RW allergy.