We treated 31 patients with ragweed conjunctivitis in an eight-week, double-masked parallel-group study with cromolyn sodium 4% ophthalmic solution. Analysis of data showed preseason serum IgE antibody to ragweed level was a significant predictor of drug response. Of 16 patients with ragweed IgE values of less than 100 ng/ml, the nine treated with cromolyn had significantly fewer symptoms during the course of treatment than the seven in the placebo group. Of the 15 remaining patients, who had IgE levels of more than 100 ng/ml, the eight treated with cromolyn improved but the difference between their symptom scores and those of the seven patients treated with placebo was not statistically significant.