Aim: To compare the intestinal habit of breastfed and partially or totally formula fed infants. Methods: We studied non weaned infants under 6 months of age, who had no evidence of gastrointestinal disease based upon clinical information. A survey to the mother included demographics, questions regarding stools, their frequency and characteristics as well as associated symptoms (straining, colic), feeding pattern and its changes. At the moment of inclusion the decision concerning feeds had already been taken and was not the responsibility of researchers. In a subset a second survey was performed. Results were analyzed by Student t test for nonpaired data. Results: 178 infants (93 girls) were included and distributed as follows: group A, 48 (1.3 ± 1.3 months) exclusively breastfed; group B, 67 (2.0 ± 1.2 months) breast and formula fed; group C, 63 (3.0 ± 0.6 months) formula fed. The daily frequency of defecation was 4.2 ± 1.7, 2.6 ± 1.5 (P = 0.009) and 2.0 ± 1.3 (P = 0.045 against the former, NS against the latter) in groups A, B and C respectively. Two infants (4%) in group A had associated symptoms (straining), one of them with hard stools, whilst 34 (51%) in group B had experienced uncomfortable, mostly hard stools since formula was started: In group C 44% (28) presented with straining and difficulty in passing dry stools. In the second survey, performed 1.8 ± 0.5 months later, infants in group A passed 2.8 ± 1.9 per day; however they were described as loose or soft in the same proportion as initially (95%). In group B, stools were significantly less and their characteristics were the same as before. Changes in intestinal habit in group C were more when compared with group A (no matter which formula) but the distribution of commercial formulae was not homogeneous in order to analyze in equity. Conclusions: Introduction of milk formulae (perhaps some more than others) influences infant's intestinal habit and is frequently associated with uncomfortable stools.