Background: Renal itch is a relatively common and distressing problem for patients with chronic renal failure. Ondansetron, a serotonin type 3 receptor antagonist was developed for relief of chemotherapy induced nausea. Recently, anecdotal reports describe relief of renal itch with ondansetron. Objectives: We performed a double-blind randomized placebo-controlled trial to objectively assess the effectiveness of ondansetron in renal itch. Patients and methods: With approval from the local ethical committee, 24 patients on haemodialysis were enrolled in the trial. On a random basis 14 patients were blindly allocated to the ondansetron-placebo sequence and 10 to the placebo-ondansetron sequence. Baseline values for itch were obtained for 7 days before the treatment period and there was a 7-day washout between the treatment periods. During the treatment patients received either 8 mg of ondansetron three times a day or a placebo tablet three times a day for 2 weeks. Patients were asked to record the severity of their pruritus on a visual analogue scale (VAS) twice a day. At the end of the study patients were asked blindly which treatment they had preferred. Results: Seventeen patients completed the trial. Pruritus decreased by 16% (95% CI: 0.5-32%) during active treatment and by 25% (95% CI: 9-41%) during treatment with placebo. The change in VAS scores during treatment with ondansetron (P = 0.04) and placebo (P = 0.01) were both significant. Eleven patients expressed a preference, seven for placebo and four for ondansetron. Conclusions: Our results show that ondansetron is no better than placebo in controlling renal itch.