A patient undergoing evaluation for malignant melanoma was thought to have a metastatic process involving the anterior mediastinum, axilla, spleen, and possibly liver based on radiologic findings from positron emission tomography and computed tomography scans. The clinical picture did not corroborate this suspicion, and biopsies ultimately confirmed lymphangioma in the accessory spleen and subcutaneous tissues, leading to a diagnosis of lymphangiomatosis. Diagnosis and management of lymphangiomatosis is clinically challenging. This report reviews the literature on the pathology, diagnostic imaging, and management of lymphangiomatosis.