Thirty-eight patients with seasonal rhinitis due to grass pollen allergy took part in this double-blind study comparing 1% nedocromil sodium and placebo. Treatment was allocated by randomised coding sheet and consisted of 1% nedocromil sodium nasal spray or placebo given four times daily for four weeks during the peak pollen season. Highly significant (p less than 0.001) differences in favour of nedocromil sodium were seen for all signs and symptoms recorded at clinical assessments, and for diary card assessments of symptoms. In addition, time to effect and patient and clinician opinions of treatment significantly (p less than 0.001) favoured nedocromil sodium. Laboratory data on blood and urine samples taken before and after treatment showed no significant effects, and both treatments were well tolerated. Nedocromil sodium 1% nasal spray taken four times daily was shown to be an effective treatment for grass pollen rhinitis.