Mesenchymal hamartoma (MH) is a rare but distinctive tumor-like malformation of the liver with predilection for infants in the first 2 years of life. MH accounted for only 5% of all primary hepatic tumors seen at Children's Hospital. Boston, over a 57-year-period from 1924 through 1981. There were nine children (five females, four males) with a median age of 11 months at the time of diagnosis (range: 5 months to 7 years). The most common presenting manifestation was abdominal enlargement with a palpable nontender mass. Exploratory laparotomy was performed in each case. The MHs were located in the right lobe and varied in size from 3 to 21 cm (average diameter. 16 cm). One MH also involved a portion of the left lobe. Four of seven children who underwent hepatic lobectomy were alive and symptom-free for 1 to 18 years later. There were three intraoperative deaths due to hemorrhage and cardiac arrest. One child underwent biopsy only with drainage of cyst fluid but died in the postoperative period because of pneumonia. The last child who had wedge resection of an incidental MH died of extrahepatic biliary atresia. Although complete surgical resection is curative, past experience indicates a substantial risk for operative mortality. In some cases, a more conservative approach to therapy is suggested with partial excision or drainage of cysts and possible resection at a future date.