In an outbreak of waterborne giardiasis where 1300 subjects were diagnosed, with Giardia lamblia, 139 continued to have abdominal symptoms of whom two of three had negative stool culture and microscopy. These were considered to have a postinfectious functional gastrointestinal disorder. We investigated visceral hypersensitivity in patients with persisting abdominal symptoms after Giardia infection and assessed the effect of 5HT(3)-antagonist ondansetron. Twenty-two patients with Giardia negative stools and 19 controls were included. A subset of patients (n = 15) had both irritable bowel syndrome (IBS) and functional dyspepsia (FD). All subjects underwent a satiety test with a soup combined with three-dimensional ultrasound. Fifteen of 22 patients underwent double-blind, randomized, placebo-controlled study with the 5-HT(3) antagonist ondansetron given orally. Drinking capacity was lower in patients than in controls (P < 0.01) and gastric emptying was reduced (P < 0.05). Patients had more symptoms both fasting and postprandially (P < 0.001) compared to controls. Ondansetron had no effect on these parameters except from less nausea postprandially (P < 0.05). In conclusion, patients with Giardia-induced gastrointestinal symptoms developed both IBS and FD. They exhibited gastric hypersensitivity with lower drinking capacity and delayed gastric emptying. The 5-HT(3) antagonist ondansetron did not improve drinking capacity, gastric emptying or symptoms except nausea.