A double-blind, randomized, placebo-controlled, parallel trial was conducted to compare the efficacy and safety of terfenadine, 60 mg (immediate-release)/pseudoephedrine hydrochloride, 120 mg (controlled-release) (T/Ps) and clemastine fumarate, 1.34 mg (immediate-release)/phenylpropanolamine, 75 mg (sustained-release) (C/Ph) in a combination tablet b.i.d. in 178 patients (12-59 years of age) with symptoms of seasonal allergic rhinitis. After seven days of treatment, the total symptom scores recorded in the diaries of 175 patients showed that both therapies had a highly significant overall treatment effect when compared with placebo (P < or = .02). The overall level of improvement, as well as improvement of individual symptoms, was similar with the two therapies. Total symptom scores assigned by physicians to 170 patients showed significant and similar levels of improvement with both therapies when compared with placebo (P < .01). The two therapies were also similar on physicians' evaluations of overall effectiveness. Both therapies relieved most histamine-mediated symptoms as well as nasal congestion, although only T/Ps showed improvement of the latter symptom in both the patients' diaries and physicians' evaluations. Among 178 patients, drowsiness and fatigue occurred more often in the C/Ph group (25% and 11.7% for the two adverse events, respectively) than in the T/Ps group (10.2% and 1.7%, respectively). The incidence of insomnia and dry mouth/nose/throat was higher with T/Ps (23.7% and 11.9%, respectively) than with C/Ph (6.7% and 3.3%, respectively). No serious or unexpected adverse events were reported. These results indicate that T/Ps and C/Ph are both superior to placebo and equally effective in the treatment of symptoms of seasonal allergic rhinitis.(ABSTRACT TRUNCATED AT 250 WORDS)