Detailed histological and histochemical studies using PAS, AB-PAS, and high-iron diamine-AB were done on 12 alimentary tract duplication cysts in children. Nine of these were thoracic and three were abdominal in location. A total of 11 cysts (nine thoracic and two abdominal cysts) were lined by gastric mucosa seen in varying stages of maturation. One triloculated cyst had, in addition, small intestinal mucosa and bronchial wall mucosa. One intra-mucosal rectal cyst was lined exclusively with primitive rectal mucosa. The cysts showed a variable pattern of mucin histochemistry. In three cysts, the gastric mucosa showed neutral mucins. In another three cysts, in addition to neutral mucins, focal positivity for sulphomucins was observed in the apical region of the surface epithelial cells. In the remaining five cysts, the gastric mucosa showed neutral mucins along with a variable mixture of sulpho- and sialomucins. A correlation was seen between mucin histochemistry and the age of the patient. Neutral mucins or focal positivity for sulphomucins was seen in patients aged 7 months or more, whereas a mixture of sulpho-, sialo-, and neutral mucins was identified in patients who were 1 month old, or less.