Background: Irritable bowel syndrome (IBS) is a common gastrointestinal disorder. Serotonin type 3 (5-HT₃) receptor antagonist alosetron hydrochloride is indicated for women with chronic, severe diarrhea-predominant IBS who have not responded adequately to conventional therapy. However, whether or not the therapeutic efficacy of 5-HT₃ receptor antagonists has gender difference is uncertain. Methods: A double-blind, placebo-controlled, parallel-group, comparative study was conducted to evaluate the effect of novel 5-HT₃ receptor antagonist, ramosetron hydrochloride, in male and female patients with diarrhea-predominant IBS. 418 subjects were randomized (109 subjects: placebo, 105 subjects: 1 µg, 103 subjects: 5 µg, and 101 subjects: 10 µg) and administered the study drug once daily. Results: The monthly responder rates of ‘Patient-reported global assessment of relief of irritable bowel syndrome symptoms’ in the 5- and 10-µg ramosetron hydrochloride-administered groups were higher than the placebo group (26.92, 42.57, and 43.01% for placebo, 5 and 10 µg). Moreover, the difference of the responder rate in comparison with the placebo group was similar in males and females. As for safety, there was tolerability at doses up to 10 µg. Conclusion: Ramosetron is an effective and well-tolerated treatment not only for female IBS patients but also for male patients.