OBJECTIVE--To evaluate histologic and gross features of mesenchymal hamartoma of liver and similar lesions in relation to determining likely pathogenesis. DESIGN--Case series of patients presenting to a tertiary care hospital over 16 years. PATIENTS--Three children with mesenchymal hamartoma ranging from newborn to 11 months of age and one 12-year-old girl with torsion of an accessory lobe of liver. MAIN OUTCOME MEASURES--Similarity of gross vascular and segmented anomalies as well as apparent consequent histologic features of mesenchymal hamartoma to those of torsion of an accessory lobe of liver indicate that they are pathogenetically related. RESULTS--Dissecting microscopic examination revealed a single vascular supply in one case and remote thrombi in two cases of mesenchymal hamartoma evaluated. The histologic features of mesenchymal hamartoma (hypocellular central zone and hypercellular periphery) were duplicated in the lobe of liver with torsion. CONCLUSIONS--Mesenchymal hamartoma represents a lesion with an anomalous solitary vascular supply that may evolve into its specific pattern with stromal cysts as a result of early ischemic changes.