Colon cancer patients are more likely than healthy subjects to excrete breath methane, and methane production has been suggested as a screening method to predict risk of developing colon cancer. To assess whether methane production is related to fiber ingestion and bowel function, we measured end-alveolar breath methane in 126 healthy subjects; 36% produced methane (>3 ppm). Fifteen methane producers and nine controls were selected to participate in a 17-day study: a self-selected diet for 7 days, followed by the same diet plus 24 g/day dietary fiber (Fiber One Cereal) for 10 days. Methane was measured three times per day on three days during each diet. Two subjects screened as methane producers stopped excreting methane during the study, and one control started to excrete methane. Radiopaque pellets were given at the start and end of each diet, and feces were collected, homogenized, and dried. Mean transit time (p < 0.0001) and fecal pH (p < 0.0003) were significantly decreased with fiber, whereas fecal hydration scale (p < 0.003) fecal dry (p < 0.0001) and wet (p < 0.0001) weight, and frequency of defecation (p < 0.0006) were significantly increased with fiber. No significant change in methane excretion (p < 0.057) was seen with added fiber. Thus, in this study methane excretor status was not constant in healthy subjects and was not significantly changed by 24 g/day dietary fiber as Fiber One Cereal.