Background: While fibre is believed to cause gaseous symptoms, a study in healthy volunteers showed no increase in flatulence when the diet was supplemented with fermentable (psyllium) or non-fermentable (methylcellulose) fibre. However, extrapolation of this observation to subjects who use fibre is arguable since these individuals may have a propensity to gaseousness. In the present study, gaseous complaints during fibre ingestion were assessed in subjects who believed that a previous exposure to fibre induced gas, Methods: In a double-blind protocol, subjects were randomized to one of four treatment periods, during which the regular diet was supplemented for 1-week periods with two daily doses of placebo 10 g, psyllium 3.4 g, methylcellulose 2 g or lactulose 5 g. A symptom diary was maintained for 1-week periods on or off treatment. Results: During treatment, the lactulose group passed gas significantly more often than did the psyllium or the methylcellulose group (P = 0.01). No other symptom was significantly different among the treatment groups. Conclusions: (1) psyllium and methylcellulose did not cause greater gaseous symptomatology than did placebo in subjects who believed that these preparations caused gas; and (2) subjects commonly misidentify dietary components that cause gaseous symptoms.