Sclerosing mesenteritis is a rare non-cancerous disease affecting the small bowel mesentery, which occurs predominantly in older age. The exact etiology is not known. Clinical symptoms are very variable and they include abdominal pain and distention, nausea, and vomiting. We present a case of a 4-month-old boy, who died suddenly at home. Based on the investigation of the circumstances of death, results of toxicology tests and histological examination the case was concluded as sudden infant death syndrome. During life, the child had repeated abdominal pain with distention. Secondary autopsy finding was a significant thickening of terminal ileum mesentery the length of 2,5 cm, with thickness reaching up to 1,5 cm and affecting almost half the circumference of the small intestine. According to the current literature, the case discussed in this article is the youngest recorded.