Eighty-nine pediatric patients aged 6 weeks to 20 years (mean, 3.8 years) who underwent Nissen fundoplication were reviewed. Follow-up, including upper gastrointestinal (GI) series, was obtained in 55 patients (61.8%). Fifteen patients developed paraesophageal hernia (PEH) (16.8%). PEH was diagnosed between 4 and 36 months following fundoplication (mean, 17 months). Patients were divided into three groups: A, those with significant mental dysfunction (45); B, those with previous tracheoesophageal fistula (12); and C, others (32). Incidence of PEH is 20% for group A, 16.8% group B, and 12.5% group C. Combining groups B and C, 5 of 25 patients (20%) who underwent fundoplication at less than 1 year of age developed PEH, whereas one of 19 older patients (5.3%) developed PEH. One of 25 patients (4%) who had crural repair at fundoplication developed PEH, whereas 14 of 64 patients without crural repair (21.9%) developed PEH. At surgery, PEH occurred at the left posterolateral aspect of esophagus. We conclude that (1) follow-up after fundoplication should continue for 36 months and include upper GI series; (2) patients under one year of age undergoing fundoplication may be at a higher risk for PEH; and (3) technical refinement including crural repair may be required to prevent PEH.