Meniere's disease affects a person's sense of balance. An attack can last 20 minutes to 2 hours or longer. Symptoms include rotational vertigo, hearing loss, tinnitus and a sensation of fullness in the affected ear and may be associated with nausea and vomiting. One hypothesis is that Meniere's disease is caused by the excessive accumulation of fluid in the balance tubes within the inner ear. Sildenafil may alleviate the symptoms due to its vasodilatory activity. The purpose of this study is to assess the safety and efficacy of sildenafil (Viagra) compared with placebo on symptoms during one acute attack.