The efficacy of the new anti-allergic compound oxatimide was tested in a double-blind study against a placebo during the pollen season of 1976 in 215 patients (4 1/2--69 years) with hay fever. Adult patients were instructed to take two tablets (= 20 mg) three times a day for 2 weeks, children received one tablet/10 kg of body weight daily; in addition, they were given a supply of diphenhydramine as a potential supplement. Twelve patients, six from either group, were excluded from the analysis. A significantly higher proportion of patients from the active drug group (n = 104) experienced no or only a few days of complaints, i.e. rhinitis and conjunctivitis, as well as asthma-like complaints, as compared with those of the control group (n = 99). Also, diphenhydramine consumption became significantly lower in the active drug group from the second day onwards, whereas a gradual increase and continuous need were seen in the greater part of the control group. One week of treatment appears sufficient to judge whether the individual patient benefits from the drug or not.