The signature non-ocular feature of this syndrome is the occurrence of numerous, sometimes thousands, of gastrointestinal polyps located mainly in the colon. Precancerous colonic polyps generally develop by the age of 16 years. Many of these eventually show malignant changes with colorectal cancer developing in nearly all patients by the sixth decade of life but the mean age of colon cancer diagnosis is at 39 years. Extracolonic lesions such as desmoid tumors, sebaceous cysts, osteomas, fibrosarcomas, and other tumors are frequently seen. Patients are also at higher risk of thyroid, adrenal, and liver cancer.
Dental anomalies, soft tissue tumors, and jaw osteomas are prominent features found in a variant of FPC known as Gardner syndrome.