Fructo-oligosaccharides (FOS) may have potential benefits, since they exhibit many soluble dietary fibre-like properties. Foods currently available for weaning infants are generally low in fibre content and lack these potential benefits. Data documenting tolerance of FOS in weaning foods are greatly lacking. Our present objective was to evaluate the tolerance and gastrointestinal effects of FOS-supplemented infant cereal used as a daily addition to the diet of healthy infants. Healthy infants were randomly assigned to receive either 0.75 g FOS per serving of cereal or placebo for 28 d. The primary outcome of interest was gastrointestinal tolerance, which was assessed by daily parental reporting of functional variables for 28 d, including stool patterns and signs and symptoms of gastrointestinal tolerance. Secondary outcomes were also measured including: cereal intake (g cereal and g FOS/d), stool pH, changes in anthropometric measurements and adverse events. The study population included a total of fifty-six infants, age range 16.2-46.2 weeks with a mean age of 32.5 (sd 8.9) weeks; twenty-nine infants were randomized to the control group (age 31.8 (sd 9.0) weeks) and twenty-seven to the FOS-supplemented group (34.7 (sd 8.9) weeks). Average daily total intake per infant and average intake per serving were similar in both groups. Average FOS consumption was 0.74 (sd 0.39) g/d and as high as 3.00 g/d. Stool consistency was less likely to be described as 'hard', and more likely to be described as 'soft' or 'loose', in the FOS v. control group. The mean number of stools per infant was 1.99 (sd 0.62) per d in the FOS-supplemented group compared with 1.58 (sd 0.66) in the control group (P=0.02). There were no differences between the groups in reporting for crying, spitting-up or colic. No differences were found for stool pH. FOS-supplements added to cereal were well tolerated in doses of up to 3.00 g/d. FOS consumption led to more regular and softer stools, without diarrhoea, as well as less-reported frequency of symptoms associated with constipation such as hard stools or skipped days without stool. The present study is one of few studies documenting tolerance to increased fibre intake in the form of FOS as part of a weaning food.