The bacterial contamination of infant weaning foods was examined in the context of a longitudinal study of lactation and infant growth, the Chiang Mai Lactation Study. Sixty-two mother-infant pairs were selected by random sampling from a rural area outside the city of Chiang Mai and studied for 48 hours in their homes on six occasions over the first year of life. Data on food hygiene practices and maternal factors were related to the total bacterial count per gram and coliform content of weaning foods. Bottle feeding, premastication, and mashing were significantly related to an increased bacterial content of weaning foods, while boiling foods to make soups, preparing in and feeding from a banana leaf, and using boiled water to prepare foods all reduced their bacterial content. Storage also increased the bacterial contamination in foods and foods were more highly contaminated in the rainy season. Maternal age and education were also related to some feeding practices. By promoting the feeding of traditional, but less contaminated weaning foods, an intervention is put forward which would aim to reduce weaning food contamination and thereby reduce incidence of diarrhoea in this area of Northern Thailand.