Before carrying out a breastfeeding promotion programme in a periurban area of Mexico City, we conducted a rapid ethnographic study to determine the factors associated with absence of exclusive breastfeeding. The responses to pilot interviews were used to develop a standardized questionnaire regarding reasons for infant feeding choice, sources of advice, and barriers to breastfeeding. We interviewed a random sample of 150 mothers with a child < 5 years of age; 136 (91%) of them had initiated breastfeeding; but only 2% exclusively breastfed up to 4 months. The mothers consistently stated that the child's nutrition, health, growth, and hygiene were the main reasons for the type of feeding selected; cost, comfort, and the husband's opinion were less important. Physicians were ranked as the most important source of advice. Reduction or cessation of breastfeeding occurred on the doctor's advice (68%); or when the mothers encountered local folk illnesses such as "coraje" (52%) or "susto" (54%), which are associated with anger or fright; or had "not enough milk" (62%) or "bad milk" (56%); or because of illness of the mother (56%) or child (43%). During childhood illnesses and conditions, breastfeeding was reduced and the use of supplementary foods was increased. This study emphasizes the importance of cultural values in infant feeding choices, defines specific barriers to breastfeeding, and provides a basis for interventions to promote exclusive breastfeeding in the study population.