OBJECTIVE: Evidence of the effects of tight, prolonged binding of infants on development is inconclusive and based on small ethnographic studies. The null hypothesis was that Mongolian infants not swaddled or swaddled tightly in a traditional setting (to >7 months of age) do not have significantly different scores for the Bayley Scales of Infant Development, Second Edition (BSID-II). PATIENTS AND METHODS: In a randomized controlled trial, 1279 healthy newborns in Ulaanbaatar, Mongolia, were allocated at birth to traditional swaddling or nonswaddling. The families received 7 months of home visits to collect data and monitor compliance. At 11 to 17 months of age, the BSID-II was administered to 1100 children. RESULTS: No significant between-group differences were found in mean scaled mental and psychomotor developmental scores. The unadjusted mean difference between the groups was -0.69 (95% confidence interval [CI]: -2.59 to 1.19) for psychomotor and -0.42 (95% CI: -1.68 to 0.84) for mental scores in favor of the swaddling group. A subgroup analysis of the compliant sample produced similar results. BSID-II-scaled psychomotor and mental scores were 99.98 (95% CI: 99.03-100.92) and 105.52 (95% CI: 104.89-106.14), respectively. Background characteristics were balanced across the groups. CONCLUSIONS: In the Mongolian context, prolonged swaddling in the first year of life did not have any significant impact on children's early mental or psychomotor development. Additional studies in other settings need to confirm this finding. The Mongolian infants in this trial had scaled BSID-II mental and psychomotor scores comparable to United States norms.