Treatment of retrosternal diaphragmatic (Morgagni) hernia is composed of a simple surgical closure of the retrosternal opening either conventionally by open abdominal or thoracic approaches, or more recently, by using minimal access surgery (MAS). Clinical experience using the latter approach is very limited in children. Removal of the hernia sac is a controversial issue, since the sac is said to carry the risk of cyst formation or show spontaneous resolution. This issue represents a 7-year-old boy with Morgagni hernia that was successfully repaired by the MAS approach. The hernia sac was not resected because of adhesion to the adjacent tissues. The patient was readmitted with fluid accumulation within the remaining sac 2 months after the operation. A complete spontaneous resolution was observed within 2 months under conservative follow-up. To our knowledge, the temporary complication described in this paper has not been published in the literature.