Intranasal administration of a 4% solution of cromolyn sodium for the treatment of ragweed hay fever was tested in an 8-week double-blind matched-pair study involving 66 patients. Patients on active drug received 5.2 mg into each nostril 6 times daily; control patients received a placebo spray. The treated group showed a significant reduction in mouth breathing (p less than 0.001), stuffy nose (p less than 0.002), runny nose (p less than 0.003), and postnasal drip (p less than 0.035). Patients receiving the active drug also reported fewer sneezing episodes (p less than 0.003) and nose blowing episodes (p less than 0.015). One patient using cromolyn solution developed nasal ulceration, tongue swelling, coughing, and wheezing. Other side effects were minimal and occurred with equal frequency in both groups. In the treated group relief of symptoms was most marked in patients with high preseasonal levels of IgE ragweed antibody. Intranasal 4% cromolyn solution appears to be an effective drug for the treatment of ragweed hay fever; measurement of the preseasonal level of IgE ragweed antibody is a useful screening test to identify patients most likely to achieve a maximal beneficial response to treatment.