Fibrous hamartoma of infancy (FHI) is a benign mesenchymal tumor histologically characterized by a mixture of intersecting fascicles of fibroblasts/myofibroblasts in collagenous stroma, nests of primitive oval or stellate cells in basophilic mucoid stroma, and mature adipose tissue. We hypothesized that FHI, because of histologic overlap with mesenchymal overgrowth tumors seen in CLOVES (Congenital Lipomatous Overgrowth with Vascular, Epidermal, Skeletal anomalies) and Proteus syndromes, may harbor mutations in signaling pathways associated with cellular proliferation. Formalin-fixed paraffin-embedded material from a discovery set of 4 cases of FHI was investigated by targeted next-generation sequencing of a panel of cancer-associated genes. The results were confirmed by targeted Sanger sequencing of EGFR exon 20. A validation set of 8 cases of FHI and 10 cases of other pediatric fatty tumors were investigated by targeted Sanger sequencing of EGFR exon 20. All 12 cases of FHI, and none of the 10 control tumors, showed EGFR exon 20 insertion/duplication mutations. This is the first report of molecular aberrations in FHI. The consistent occurrence of EGFR exon 20 insertion/duplication mutations in 100% of cases of FHI studied suggests that they must play a principal role in the pathogenesis of FHI, likely by conferring a potential for growth and local infiltration. Although surgical treatment will remain the mainstay of FHI treatment, tyrosine kinase inhibitors may have an adjunctive role in cases that are difficult to resect.