Background Among infants with isolated cleft palate, whether primary surgery at 6 months of age is more beneficial than surgery at 12 months of age with respect to speech outcomes, hearing outcomes, dentofacial development, and safety is unknown. Methods We randomly assigned infants with nonsyndromic isolated cleft palate, in a 1:1 ratio, to undergo standardized primary surgery at 6 months of age (6-month group) or at 12 months of age (12-month group) for closure of the cleft. Standardized assessments of quality-checked video and audio recordings at 1, 3, and 5 years of age were performed independently by speech and language therapists who were unaware of the trial-group assignments. The primary outcome was velopharyngeal insufficiency at 5 years of age, defined as a velopharyngeal composite summary score of at least 4 (scores range from 0 to 6, with higher scores indicating greater severity). Secondary outcomes included speech development, postoperative complications, hearing sensitivity, dentofacial development, and growth. Results We randomly assigned 558 infants at 23 centers across Europe and South America to undergo surgery at 6 months of age (281 infants) or at 12 months of age (277 infants). Speech recordings from 235 infants (83.6%) in the 6-month group and 226 (81.6%) in the 12-month group were analyzable. Insufficient velopharyngeal function at 5 years of age was observed in 21 of 235 infants (8.9%) in the 6-month group as compared with 34 of 226 (15.0%) in the 12-month group (risk ratio, 0.59; 95% confidence interval, 0.36 to 0.99; P=0.04). Postoperative complications were infrequent and similar in the 6-month and 12-month groups. Four serious adverse events were reported (three in the 6-month group and one in the 12-month group) and had resolved at follow-up. Conclusions Medically fit infants who underwent primary surgery for isolated cleft palate in adequately resourced settings at 6 months of age were less likely to have velopharyngeal insufficiency at the age of 5 years than those who had surgery at 12 months of age. (Funded by the National Institute of Dental and Craniofacial Research; TOPS ClinicalTrials.gov number, NCT00993551.)