A 5-YEAR-OLD white boy was found to have an abdominal mass on routine physical examination by his pediatrician. He was born 5 weeks prematurely and was delivered vaginally without complications. He has since been in good health.The results of the physical examination were normal. Mild cervical adenopathy was noted consisting of a few small, nontender freely movable lymph nodes. The results of the abdominal examination revealed a distinctly palpable nontender mass extending into the pelvis.Laboratory studies disclosed the following values: hemoglobin, 118 g/L; white blood cells, 5.4×109/L; granulocytes, 3500/μL; lymphocytes, 1100/μL; monocytes, 700/μL; and platelets, 236×109/L. Abdominal sonography (Figure 1) and abdominal computed tomography (Figure 2) were performed. The mass that was found at surgery (Figure 3) was excised (Figure 4). The microscopic appearance is shown in Figure 5. DIAGNOSIS AND DISCUSSION MESENTERIC CYSTIC LYMPHANGIOMA Lymphangiomas are benign, endothelium-lined cystic tumors that appear to